Antigenic properties of two representative allelic products of the major piroplasm surface protein (MPSP) of Theileria sergenti were studied. Sera from cattle infected with either of Ikeda and Chitose types of the parasite reacted strongly with homologous but weakly with heterologous recombinant antigens in immunoblotting. Monoclonal antibodies (MoAbs) produced against the both allelic products of MPSP parasites reacted only to the immunizing antigen. These results suggested that crossreactivity between two allelic products is very low inspite of relatively high homology in their amino acid sequences. Double staining of parasitized erythrocyte smear using type-specific MoAbs by an indirect immunofluorescent assay revealed that the set of MoAbs was useful for quantitative and differential detection of each type of parasite in mixed population.