Cryoultramicrotomy, an antisporozoite monoclonal antibody (2G3) and protein A-gold particles were used to determine the distribution of the protective surface antigen (Pk 42) of sporozoites of Plasmodium knowlesi and its precursors (Pk 52 and Pk 50). Gold particles bound uniformly to both the outer plasma and the inner pellicular membranes, indicating a uniform distribution of antigen on these membranes. Intracellular gold particles were frequently found to be associated with micronemes, but less frequently with rhoptries. Limited proteolysis of P. knowlesi sporozoites totally eliminated binding of label to the outer plasma membrane. In contrast, distribution of label on the inner pellicular membranes, rhoptries, and micronemes of trypsinized sporozoites was relatively unaltered. On the basis of the present and earlier data obtained upon metabolic labeling of these parasites, it is likely that the antigen associated with the intracellular organelles corresponds to the precursor polypeptides Pk 52 and Pk 50.