Mice can be partially protected against Plasmodium chabaudi by immunization with the antigen Pc96, isolated from the erythrocyte membranes of infected mice. We used a Pc96 specific monoclonal antibody to identify antigens which cross-react with Pc96 in P. falciparum, P. vivax, and P. cynomolgi. The cross-reactive molecules are antigens of Mr 155,000 in P. falciparum, Mr 220,000 in P. vivax, and Mr 200,000 in P. cynomolgi. They are located in the surface membranes of infected erythrocytes. Pc96 is characterized by immunoelectron microscopy and epitope mapping.