AbstractThe Muguga and Marikebuni stocks of Theileria parva differ on the basis of cross‐protection and in their schizont antigen profile determined with a panel of parasitespecific monoclonal antibodies. The phenotype and specificity of six cytotoxic T cell clones generated from an animal immunized against T. parva (Marikebuni) were investigated. All six clones had the BoT2+ BoT4− BoT8+ phenotype, were dependent on both specific antigen and T cell growth factor for proliferation and were restricted by determinants on class I major histocompatibility complex molecules. The clones killed target cells infected with either the Muguga or Marikebuni stocks of the parasite; the target cell lines tested included T cell clones which were infected in vitro with the two parasite stocks and subsequently recloned. The specificity of these cytotoxic T cell clones contrasts with that of T cell clones generated previously from animals immunized against T. parva (Muguga), in that the latter were specific for target cells infected with the Muguga stock of the parasite. Moreover, one of the clones generated against T. parva (Marikebuni) was restricted by the same major histocompatibility complex molecule as the Muguga‐specific T cell clones. The difference in parasite strain‐specificity between the two sets of clones appears to reflect the capacities of the two parasite stocks to cross‐protect, since animals immunized against T. parva (Marikebuni) are protected against challenge with T. parva (Muguga) whereas a proportion of animals immunized with T. parva (Muguga) are susceptible to challenge with T. parva (Marikebuni). Another difference between the two sets of T cell clones was that those generated against T. parva (Marikebuni) only killed a proportion of cells of a given cell line in a 4‐h cytotoxicity assay, whereas Muguga‐specific T cells invariably kill the majority of cells. However, despite this partial killing, the clones markedly inhibited growth of parasitized cell lines when cultured with them for a period of 5 days.