Techniques have been established for the generation of bovine cytotoxic T cell lines and clones specific for lymphocytes infected with the protozoan parasiteTheileria parva. Theileria-specific cytotoxic T cell lines are generated by repeated stimulation in vitro with autologousT. parva-infected cells, of peripheral blood mononuclear cells from cattle immunized withT. parva. Theileria-specific cytotoxic T cell clones can be derived from these restimulated cultures by limiting dilution of the cells in the presence of irradiated stimulator and filler cells and T cell growth factor. The clones have the BoT4^– BoT8⁺ phenotype and are restricted by class I MHC products. Parasite strain specificity of the clones differed depending on the parasite stock used for immunization, and in some instances differed between individual animals immunized with the same parasite stock. Preliminary evidence suggests that the latter is due to an influence of the MHC phenotype of the animal. Results of the parasite strain specificity of the cytotoxic T cell response are consistent with findings of cross-immunization experiments with the two stocks of the parasite studied.