In contrast to the uncertain role of mycoplasmas in genital disease, there is no doubt that they produce respiratory disease. The first mycoplasma isolated, M. mycoidesis is the aetiological agent of bovine pleuropneumonia, a disease now confined to parts of Africa and Australia. M. capri causes a similar disease in goats which is geographically more widespread. Enzootic pneumonia of pigs, seen in various parts of the world, is due primarily to M. suipneumoniae. Although some strains of M. hyorhinis also seem capable of causing disease. Not all strains of a mycoplasma species are equally pathogenic; only some strains of M. pulmonis produce severe respiratory disease in mice and rats. Further, concomitant infection with other agents may result in more severe disease; this is particularly seen in M. gallisepticum infections of poultry. Of those mycoplasmas which infect man, only M. pneumoniae is known to cause respiratory disease, mainly involving the younger age groups. Mycoplasmas spread by close contact so that disease is seen in family groups or where there is crowding or herding. Factors in resistance to respiratory disease caused by mycoplasmas have been poorly understood but the importance of local immunity is becoming more clear.