The chemical and biological properties of the constituents of tetrathionate media have been investigated.

(1) The selective properties of thiosulphate and tetrathionate are expressed in graphic form, a method which clearly illustrates their differential effect on various intestinal organisms and is of value in preparing media to suit particular needs.

(2) Thiosulphate, tetrathionate, iodide and, indirectly, chalk all have some selective action, but the most important single constituent is tetrathionate.

(3) The discovery that certain organisms, mainly of the Salmonella group, are capable of reducing tetrathionate rapidly to thiosulphate suggests that in tetrathionate media part at least of the selective action is due to the ability of the tetrathionate to act as an alternative hydrogen acceptor to oxygen.

(4) In the light of these facts many of the defects and puzzling features of tetrathionate media can be explained, especially the frequent heavy growth of Proteus.

(5) A medium containing balanced tetrathionate shows a different kind of selectivity from media of Muller's type and has given consistent results as a routine solid medium for plating of faecal specimens.

(6) A diminution in the amount of thiosulphate in media of Muller's type is desirable, since Bact. typhosum is relatively sensitive to thiosulphate.

(7) The possible value of a biochemical approach to the study of selective media is emphasized.

It is a pleasure to thank Prof. M. E. Delafield and Prof. G. S. Wilson for constructive criticism of this paper, and we are much indebted to Dr C. Hamilton Willkie for taking the photograph.

1. A total of 499 rural and 437 urban Bantu school children were examined for their diphtheria carrier rate and Schick immunity.

2. Clinical diphtheria amongst the rural and urban Bantu was also investigated.

3. The virulent C. diphtheriae carrier rate was found to be 3·2% in the rural children and 1·8% in the urban with virulent/avirulent ratios of 1: 1·1 and 1: 2·5 respectively.

4. The Schick-positive rate in children aged 6–17 years was found to be 8·0% in rural children and 13·7 % in urban.

5. Clinical diphtheria was rarely encountered under rural conditions, but was more common in urban natives.

6. The case mortality rate in eighty-nine clinical cases was 14·4%.

7. 53% of the clinical cases occurred in the 0-5 years age group.

8. 89% of the strains recovered belonged to the mitis type. No intermediate strains were encountered.

9. The reason for the infrequency of clinical diphtheria amongst the Bantu is discussed and various theories are reviewed.

10. It is concluded that the infrequency of clinical diphtheria is partly due to the environment, but that there is also a racial factor. It is suggested that the racial factor lies in an ability to produce antitoxin quickly. There is not sufficient evidence in this investigation to show whether the racial factor is genetic, but in view of Turbott's work amongst the Maoris it is suggested that the racial factor in immunity to diphtheria may be genetic amongst the Bantu also.

I have pleasure in acknowledging the continued interest of Dr E. H. Cluver, Director, and Dr G. Buchanan, Deputy-Director, of the South African Institute for Medical Research in the progress of this work. I also wish to acknowledge gratefully the help given me by Dr Prestwick, Dr Miller and Dr Xuma at Alexandra Township. I am indebted to the Administration, and in particular to Dr J. W. Stirling, Principal Medical Officer of Bechuanaland Protectorate Government, for permission to carry out the investigation at Kanye. To Dr Marcus of the Seventh Day Adventist Mission, Kanye, I am deeply indebted for help and hospitality, and to Chief Bathoën who assisted me in making contact with the requisite number of school children at Kanye. My thanks are also due to the Mother Superior and Sisters of the Holy Cross Mission, Alexandra Township, and the many Bantu school teachers who gave me access to the children in their charge. Mr Barnes (S.A.I.M.R.) very kindly carried out the statistical tests of Tables 1 and 2. The travelling expenses involved in this work were defrayed by a grant from the National Research Board.

1. Dick tests on 743 Bantu children aged 6–20 years showed 5·0% positive reactors.

2. Throat swabs from 200 urban native children showed 19% carriers of Lancefield's group A Streptococcus haemolyticus.

3. The basis of Bantu immunity to scarlet fever is discussed.

I am indebted to Dr E. H. Cluver, Director, and Dr G. Buchanan, Deputy Director of the South African Institute for Medical Research, for their continued interest in the progress of this work, and I am especially grateful to Dr D. Ordman of this Institute who isolated and typed the streptococci for me. My thanks are also due to Dr Marcus, Kanye, Bechuanaland Protectorate, and to Dr Prestwich and Dr Xuma, of Alexandra Township, for making it possible to examine the requisite number of Bantu children. The travelling expenses involved in this investigation were defrayed by a grant from the National Research Board.

1. It has been shown on serological and biochemical grounds that Str. faecalis falls into Lancefield's group D. Str. liquefaciens is considered to be a variety of Str. faecalis, and Str. glycerinaceus (Orla Jensen) has insufficient claim to a separate name.

2. Str. lactis has, on biochemical and serological grounds, been clearly differentiated from Str. faecalis and the other members of group D.

3. Reasons are given for assigning Str. lactis to a new serological group ‘N’ into which Str. cremoris also falls.

An apparatus which prevents the distribution of infective droplets into the air of the laboratory during the intranasal inoculation of mice is described.

Experiments using Chr. prodigiosum as indicator organism are described to show the extent and duration of the droplet dissemination by inoculated mice, and the effect of the apparatus in reducing this dissemination.

We wish to express our thanks to Dr R. B. Bourdillon for many suggestions in the design of the apparatus, and to Mr C. D. Sutton for the photographs.

Experiments are described for determining the effect of hypochlorite mists and hypochlorous acid gas on aerosols of influenza A virus.

The virus aerosol is shown to be at least as easily killed by hypochlorite mists as Streptococcus salivarius. It is also killed by liberating hypochlorous acid gas itself into the atmosphere. A concentration of 1 vol. of gas in 2 million vol. of air is probably effective in destroying 99% or more of virus particles when the proportion of these in the air is small.

Preliminary experiments on mice and cats are recorded which failed to reveal any toxic effects produced by inhaling the gas in relatively high concentrations or for prolonged periods. Acute irritation of mucous membranes only was found. This did not appear to lead to any increased susceptibility of mice to subsequent infection with influenza.

We wish to acknowledge our indebtedness to the late Sir Patrick Laidlaw and to Dr R. J. V. Pulvertaft in co-operation with whom the earlier experiments were made. We also wish to thank Dr W. J. Elford, Dr F. C. MacIntosh, Dr R. K. Callow and Dr T. S. Work for their assistance and advice in parts of the investigation.

Seven more carriers, all possessing appreciable quantities of Vi agglutinins, are added to those already known.

Vi agglutination tests performed on 2526 inhabitants of the Transvaal, both native and white, showed that 5·3 % had similar quantities of Vi agglutinins.

A fair number of these positive reactors were further examined for the presence of typhoid bacilli in their excreta, but the number of positive findings was very small.

It is argued that there are a large number of typhoid faecal carriers who are such minimal excreters that the typhoid bacilli in their stools escape detection.

We wish to thank Dr T. Lötter, Acting Medical Officer of Health of Pretoria, and his staff, for their kindly help in collecting the material for this paper. Our special thanks are due to Lieut.-Col. H. Nelson, Medical Officer of Health of Pretoria, now on active service, for his never-failing interest in our work which has been such a great encouragement.

1. The antigenicity of batches of T.A.B.C. vaccine after mixture with tetanus toxoid and held at 18–25° C. for 6–7 weeks, 12–13 months and 16 months was tested in groups of adult males in whom the pre-inoculation titres for the various antigens were strictly comparable.

2. The batch used within 6–7 weeks of production produced the greatest effect, but the older batches were not greatly reduced in potency.

3. Within the limitations imposed by the restricted investigation, the more effective result following the use of the 16 months vaccine as compared with the 12–13 months vaccine indicates that factors in addition to that of time were concerned in the deterioration of antigenic power.

We are indebted to the Medical Officers in charge of the Training Establishments concerned, Surgeon Captain H. F. Briggs, M.B., Ch.B, F.R.C.S. (Ed.), R.N., Surgeon Captain W. P. Vicary, M.R.C.S., L.R.C.P., R.N., and Surgeon Commander H. A. Shellwell, M.R.C.S., L.R.C.P., R.N.V.R., for their ready and valuable co-operation.

The α-naphthol modification of the Voges-Proskauer (v.-p.) test described by Barritt (1936) has proved satisfactory for the examination of the coli-aerogenes group in the hands of several workers, including Harold (1936), Vaughn, Mitchell & Levine (1938), Kluyver & Molt (1939), Iyer & Raghavachari (1938–9), and Batty-Smith (1941).

We have investigated, with the most satisfactory results, the possibility of using sulphur dioxide (Williams, Waterman, Keresztezy & Buchman, 1935) for the selective destruction of thiamin in foodstuffs.