Two monoclonal antibodies, which prevent merozoites attaching to or invading erythrocytes, react with the same or closely apposed epitopes on a minor 66 kDa Plasmodium knowlesi antigen. The antigen is processed, at the time of schizont rupture and merozoite release, to 44 and 42 kDa molecules which are present on the merozoite surface [Deans, J. A. et al. (1984) Mol. Biochem. Parasitol. 11, 189-204]. The monoclonal antibody-defined epitope, which is expressed only once on the 66 kDa molecule, is formed by a tertiary folding of the polypeptide chain (minimum size 42 kDa). The conformation of the epitope is maintained by weak intramolecular forces of attraction, rendering the epitope extremely labile; it is completely destroyed by treatment with sodium dodecyl sulphate (SDS). Polyclonal monospecific antiserum raised against SDS-treated antigen did not inhibit parasite proliferation whereas a polyclonal antiserum raised against native antigen was inhibitory. It is postulated that the monoclonal antibody-defined antigenic determinant is crucial for merozoite invasion.