The aim of this article is to review the role of monoclonal antibodies as a tool in malaria research. In particular we concentrate on a monoclonal antibody directed against a functional epitope on the merozoite surface. This antibody blocked the invasion of merozoites into the red blood cells by causing agglutination of the parasite. The antigenic determinant recognized by this mcAb, 13C11, is located on a major 230 K dalton glycoprotein of the mature Plasmodium knowlesi parasite. Using the 13C11 monoclonal antibody it was possible to follow the synthesis and insertion of this glycoprotein into the schizont membrane. The relation between the intraerythrocyte schizont and the extraerythrocyte merozoite components recognized by the 13C11 were defined. Furthermore, 13C11 was used to analyze the processing of the 230 Kd plasma-membrane component to lower molecular weight products which were present on the surface of the invasive merozoite and which might play a role in the invasion process.