Hysterectomy-produced, colostrum-deprived pigs were injected twice with formalinized antigen prepared from the J strain of Mycoplasma suipneumoniae; the first injection was with Freund's adjuvant and the second injection without adjuvant. The immunity of these animals was tested by inoculating them intranasally with different doses of lung suspension prepared from cases of enzootic pneumonia. Two of the pigs were not killed shortly after infection, but were kept and challenged with enzootic pneumonia in order to compare the serology of experimentally-injected animals with the serology of the immune state following the experimentally-induced disease.

In a second main experiment, a pregnant sow was injected twice with nonformalinized antigen without adjuvant, and her litter was subsequently exposed to the disease at 7 days of age after suckling naturally from birth.

There was no evidence to suggest that the injections had protected the pigs in the first experiment against a high dose of infection, but they may have given some protection against low doses. The piglets suckled by the injected sow were not protected against two different doses of infection.

Serum samples taken at different stages were examined by the metabolic inhibition (MI) test, the indirect-haemagglutination (IHA) test, the complement fixation (CF) test and the gel-diffusion precipitin test, using M. suipneumoniae as antigen.

Serum samples taken before injection in the first experiment were all negative in the MI test and they became positive after the injections of antigen. However, 1 the highest MI titres obtained were not associated with obvious immunity; nor was the development of true immunity after experimental infection associated with a change in MI titre.

In the first experiment, substantial IHA titres (over 20,000) were recorded by 14 days after the second injection of antigen. Again, there was no correlation between the IHA titres and the area of pneumonia following experimental infection. In the sow experiment, IHA titres developed after the first injection and increased after the second; a high IHA titre occurred in the colostrum and titres of 320 or more were present in the piglets 7 days after birth.

The CF titres appeared earlier than the IHA titres but did not increase so markedly thereafter. Once more, there was no correlation between the titre before infection and the area of pneumonia afterwards.

In the gel-diffusion test, precipitins were demonstrated in all the post-injection serum samples tested, most of the samples being positive after the first injection. Precipitins were also demonstrated in the colostrum of the injected sow and in her uninjected litter at 7 days of age.

From these experiments it was concluded that, as judged by the development of pneumonic lesions and in marked contrast to the known immunizing effect of the disease itself, injections of antigen given in this manner had little or no protective effect against the dose levels of infection used. Nevertheless, the titres obtained in the MI, IHA and CF tests were comparable with those obtained earlier in pigs that were strongly immune, which provides further evidence for the suggestion that these tests do not measure protective immunity.

Miss Elaine Repworth provided technical assistance. Most of the expenses of this work, including the salary of two of the authors (R. G. H. and R. L. W.) were met by a grant from the Agricultural Research Council.