The pathogenesis of infection and oncogenesis by feline leukemia virus and factors determining host resistance to FeLV will be studied in specific pathogen-free cats. Young cats (less than or equal to 8 weeks of age) that are highly susceptible to FeLV and adult cats that have substantial autologous resistance to FeLV will be infected by either oral-nasal or intraperitoneal inoculation. The events in the genesis of progressive (leukemogenic) vs regressive (immunizing) infection will be studied principally by immunofluorescence. Emphasis will be placed on the phase of early viral containment by cells of the mono-nuclear phagocyte and lymphoid series in regional lymphoid tissues. The importance of macrophage function, resistance to FeLV, and the influence of FeLV infection in vivo and in vitro on non-specific and specific macrophage functions will be investigated.