Culture of a lymphoblastoid cell line infected with the macroschizont stage of the protozoan parasite Theileria annulata at 41 degrees C induces differentiation to the next stage, the merozoite. We have demonstrated that this development results in the loss of monoclonal antibody epitopes associated with the macroschizont stage, and the appearance of epitopes associated with the piroplasm (the intra-erythrocytic stages). One of the monoclonals (5E1) was shown by immunoelectron microscopy to bind to the surface of heat-induced culture forms which had size and structural characteristics of the merozoite. The monoclonal was found to detect two polypeptides of 30 kDa and 25 kDa in extracts of piroplasms. The 30-kDa polypeptide was also detected in a merozoite extract, but was not detected in an extract derived from macroschizont-infected lymphoblastoid cells. We conclude that the heat-induced differentiation of T. annulata in vitro results in the expression of a 30-kDa molecule which is located at the surface of the merozoite, and discuss the potential of this molecule as a component in a subunit vaccine.