Mycoplasma arthritidis is a natural cause of arthritis and subclinical infection in rats. Arthritis is characteristically self- limited but may persist in chronic form in some animals. Infected rats do not express neutralizing or opsonizing antibodies against M. arthritidis, and M. arthritidis is anti-phagocytic for murine macrophages and PMNs. However, convalescent and immunized rats are highly resistant to reinfection and challenge, respectively. Marked virulence differences have been observed for different M. arthritidis strains, but the basis for this remains unknown. The immune response to surface antigens plays a key role in this disease. The specific aim of this project is to characterize the major surface antigens involved in adherence to host tissues, resistance to phagocytosis, and virulence differences among strains. Specific antibodies produced by rats against surface antigens will also be characterized in terms of their immunoglobulin class and subclass, protective capacity, ability to block adherence, expression of neutralizing or opsonizing activity, and recognition of antigens of heterologous as well as homologous strains. Methods will include one- and two-dimensional electrophoresis, immunoblotting, radioimmunoprecipitation, serologic assays, and assays for measuring phagocytosis, opsonization, and peritoneal clearance mechanisms. This work should provide useful and broadly applicable information on host- parasite interaction at the level of the cell surface. This is the site of initial contact between host and parasite and the location of those microbial antigens which are most important in pathogenesis.