The merozoite, the extracellular form of the erythrocyte stage of the malarial parasite, invades the erythrocyte and develops intracellularly. Cloned hybridoma cell lines secreting monoclonal antibodies directed against the merozoite surface were selected by indirect immunofluorescent assay by using intact isolated merozoites. Monoclonal antibodies to a 200,000 m.w. merozoite surface antigen were selected and were used to characterize this protein and its role in erythrocyte invasion. Immunoelectron microscopy demonstrated that the antigen was located exclusively on the merozoite surface coat, distributed evenly over the entire surface. The 200,000 m.w. protein incorporated [3H]glucosamine, suggesting that it is a glycoprotein and could be purified to homogeneity by using immuno-affinity chromatography. Freshly isolated, invasive merozoites retained the 200,000 m.w. antigen, but the protein was rapidly cleaved to proteins of 90,000 and 50,000 m.w. when the merozoite was extracellular. The 50,000 m.w. fragment was retained the epitope binding to monoclonal antibody 5B1 and were labeled with [3H]glucosamine. Monoclonal antibodies against the 200,000 m.w. antigen partially inhibited merozoite invasion into erythrocytes.