The merozoite interacts in a receptor specific manner with the erythrocyte surface and is the stage against which immunity may work to block invasion. Thus, merozoite surface components are of interest for their role in erythrocyte recognition and as antigens for induction of protective immunity. We are now studying the processing of these molecules during the ultimate stages of parasite development, their role in reception and as immunogens for induction of protective immunity. The receptors on the erythrocyte (Duffy blood group determinant for Plasmodium knowlesi and P. vivax and glycophorin for P. falciparum) will be used to affinity purify the parasite antigens.