The in vitro penetration of human red cells by Babesia rodhaini will be studied with reference to the requirements for complement, immune adherence receptor, and surface antigens. Red cells with varying amounts and types of intermediate complement complexes will be asssessed for susceptibility to penetration by the protozoan. The interaction of the parasite with the complement system will be assessed. Finally, the possible requirement for complement in development of plasmodial infections will also be determined.