Hysterectomy-produced, colostrum-deprived pigs, reared in special isolation accommodation, were infected with enzootic pneumonia and later challenged with the same strain of the disease. Both the original infections and the subsequent challenges were made with intranasal inoculations of suspensions of ground pneumonic lung, but there was no evidence to suggest that any mycoplasma other than the J strain of Mycoplasma suipneumoniae was involved.

Pigs that had recovered from the disease were strongly immune to challenge, in that they developed virtually no lung lesions when inoculated with lung suspensions that produced extensive lesions of enzootic pneumonia in control animals. This was the case, even when the pigs were as young as 16 days old when first infected and were not challenged until up to 60 weeks later.

Sera from these pigs taken before infection, about 2–3 weeks after infection, at various times after natural recovery, and before and after challenge were examined using the metabolic-inhibition test, the indirect-haemagglutination test and the complement-fixation test.

The metabolic-inhibition test proved of little value, because non-specific inhibitory substances were present in the sera of some pigs both before and after infection: these substances inhibited the growth of Mycoplasma hyorhinis, Mycoplasma pneumoniae and Mycoplasma gallisepticum as effectively as M. suipneumoniae. Sometimes the non-specific inhibition was reduced by heating the sera at 56° C. for 30 min., but at other times it was not, which suggests that at least two types of non-specific inhibitors were present.

Apart from one pig, all the sera that were expected to be negative for antibodies against M. suipneumoniae proved to be so by the indirect-haemagglutination test. Titres of less than 1/5 were obtained in this test using the sera from pigs killed 12–22 days after infection, but high titres were obtained 16–60 weeks after infection. It was not possible to say whether these titres correlated with immunity.

All the pre-infection sera when examined by the complement-fixation test had titres of less than 1/10, but by 12–22 days after infection over half the serum samples had titres of 1/40 or more, and titres of 1/80–1/640 were obtained at 4 and 9 months after infection. There was some evidence to show that these titres declined more rapidly than the titres obtained in the indirect-haemagglutination test; for they were very low at 60 weeks after infection, at which time the indirect-haemagglutination titres were still high.

It seemed, therefore, that these two serological tests were measuring different aspects of the post-infection response. Also, because the complement-fixation titres were very low in two pigs that were shown to be powerfully immune, these titres did not appear to correlate with immunity.

Our work with the metabolic-inhibition test and the complement-fixation test has benefited from discussions with Dr D. Taylor-Robinson and Mr A. S. Wallis, respectively. We are grateful to Drs H. P. Chu, R. H. Leach and D. Taylor-Robinson for the reference sera and the culture mentioned in the text. 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.