SUMMARYFourteen cases of enzootic pneumonia, nearly all of which had presented diagnostic difficulties using the metabolic-inhibition test, were re-examined using specific pig antisera in the passive haemagglutination test (PHA). All proved positive forMycoplasma suipneumoniae, indicating that the test, used in this manner, might be particularly valuable for routine diagnosis.The PHA test was also used to demonstrate antibody toM. suipneumoniaein pneumonic tissue and the associated bronchial lymph nodes.To allay our concern that cross-reactions might interfere with this and other serological tests—the complement-fixation test (CF) and precipitation in agar-gel—the specificity of our reagents and the antigenic relationships ofMycoplasma hyorhinis,Mycoplasma granularum, mycoplasmaB3,Mycoplasma hyopneumoniaeand three strains ofM. suipneumoniae(including cloned and uncloned isolates of the J strain) were studied in various ways. Antibodies to medium constituents occurred in rabbit antiserum but did not present a problem with pig antisera. These antibodies were successfully absorbed from the rabbit antisera but it was not possible to remove medium constituents from the antigens used to produce antisera in rabbits by repeated washing.By all these tests, the main species of mycoplasmas studied seemed to be anti-genically distinct. No major antigenic differences between the three strains ofM. suipneumoniaewere revealed by the PHA test and the CF test; a slight difference in the precipitation lines of one of these strains (MG) in agar-gel might have indicated an antigenic variation or been a measure of some other factor.