Since the introduction of toxoid-antitoxin floccules by Glenny & Pope (1927) little has been done to improve their value as an antigen. Watson, Taggart & Shaw (1941) and Barr (1949) confirmed the work of these investigators in showing that heating at 80° C. increased their antigenic value.

It has been shown in this article that about 80% of the toxoid contained in floccules may be recovered if they are dissolved in n/20-NaOH, the alkali being allowed to act for 1 hr. before being neutralized with acid. A purity of between 0·002 and 0·0025 mg. N/Lf is obtained regularly. Such a dissolved floccule solution is a relatively poor antigen when tested in guinea-pigs, but may be rendered highly antigenic when precipitated with alum or adsorbed on AIPO4.

The method has the advantage of simplicity. It would appear that any medium capable of yielding a toxin convertible into flucculable toxoid may be used, thus doing away with cumbersome media-making methods. However, for the best results in terms of both recovery and purity, a high-value, quickly flocculating, toxoid should be used. Further, the choice of antitoxin appears to be important where recovery is concerned. With most of the antitoxins used, recoveries of between 70 and 90 % were got, although with that used, for most of the work recorded here, the recoveries were much lower.

It is not yet established which is the antigen of choice—alum-precipitated (P.D.F.) or AIPO4-adsorbed (A.D.F.) dissolved floccules. A.D.F. has the advantage of ease of preparation and of constant mineral carrier content, but further work is necessary to establish the optimum amount of carrier before a definite answer can be given. A paper on immunity responses in man will be published shortly in the Lancet.