SummaryThe protozoan parasiteTheileria annulatacauses a severe disease of cattle in tropical countries; one stage in the parasite life cycle involves the transformation of bovine lymphocytes leading to rapid lymphoproliferation. Immunity to this disease is largely cell mediated and directed against the infected lymphocyte. In this paper we report the identification of three classes of infection specific antigen (using monoclonal antibodies) one of which is found on the surface of the lymphocyte. Such antigens can be used in parasite strain typing, in providing an understanding of the molecules involved in immunity and in providing the basis for a vaccine.