The prime objective is to determine the duration and characteristics of naturally acquired immunity to M. pneumoniae infection among a cohort of 700 patients with prior M. pneumoniae infection as proven by isolation. Knowledge of natural immunity is essential to predicting the potential effectiveness and guiding the use of vaccines, which are now becoming available. The objectives include: 1) to determine the role of immunity (especially humoral, but also local and cell-mediated) following infection in prevention and/or enhancement of disease; 2) to evaluate the hypothesis that pneumonia occurs in persons over 5 years of age with M. pneumoniae infection following an earlier mild or asymptomatic sensitizing M. pneumoniae infection; 3) determination of which type of serum antibody(ies) most reliably indicate prior infection; 4) evaluation of the relation between types and titer of antibody protection from M. pneumoniae infection, and 5) observing the duration of detectable antibodies following infection. In addition to the cohort of 700 patients with history of previous M. pneumoniae infection, the infection rates among approximately 250 school children and members of 40 families are being followed longitudinally with analysis of serological specimens, collected annually. BIBLIOGRAPHIC REFERENCES: Alexander, A.G. and Kenny, G.E.: Characterization of membrane and cytoplasmic antigens of Mycoplasma arginini by two-dimensional (crossed) immunoelectrophoresis. Infection and Immunity 15:313-321, 1977. Foy, H.M., Kenny, G.E., Sefi, R., Ochs, H.D. and Allan, I.D.: Second Attack of M. pneumoniae Pneumonia. In press, J. Infect. Dis., April, 1977.