SummaryThis study examines several aspects of a polymorphic, immunodominant molecule (P!M) found in the protozoan parasite.Theileria parva.The antigen is present in allT. p. parvastocks examined, and in the related subspecies,T. p. bovisandT. p. lawrencei.It is the predominant antigen recognized by antisera from immune cattle on Western blot analysis of schizont‐infected lymphocytes, and is the only antigen which has been shown to react with anti‐schizont monoclonal antibodies (MoAbs) on Western blots or in immunoprecipitations. The antigen shows polymorphism in both size and expression of antibody epitopes among the different stocks ofT. parva.The antigen is present in sporozoites as well as schizo