Studies of factors affecting pathogenicity and of pathogenicity mechanisms are conducted by cytologic, immunologic, and biochemical methods. The specific investigations include: (A) Search for, and analysis of the factors that affect reacquisition of infectivity for mice by Trypanosoma brucei brucei maintained in vitro in the presence of tsetse organ cultures. (B) Investigation of salivarian trypanosomes with regard to: (a) interrelationships among variant-specific antigens and between common and unique anntigens among variant-specific antigens and between common and unique antigens of blood stream and culture forms; (b) specificity and identity of host proteins accreted on the surfaces of bloodstream forms; (c) nature and localization of surface carbohydrates (with the aid of plant lectins). (C) The nature, composition, and effect upon infection with Leishmania donovani of this parasite's excretory factors (EF) are being studied by immunologic and biochemical methods. (D) Studies on the relationships between antigenic composition and pathogenicity of cloned strains of T. vaginalis are conducted with the aid of gel diffusion, immunoelectrophoretic, and direct quantitative fluorescent antibody methods.