A monoclonal antibody raised against the malaria parasite Plasmodium falciparum recognised a protein of 140000 molecular weight which was synthesized during schizogony. The protein has been purified by monoclonal antibody affinity chromatography from extracts of parasitized red cells. Antibodies against the protein have been used to determine its subcellular location. The protein is not expressed on the merozoite surface and has been located in the rhoptries, the apical organelles of the merozoite.