Leucocytes from cattle with lethal or sub-lethal infections with Theileria parva were examined for responses in autologous mixed lymphocyte reactions (MLR) and for specific and non-specific cytotoxicity against cultured cell lines. During lethal infections, lymphoblasts isolated from central lymph from days 8 to 14 stimulated proliferation in autologous lymphatic lymphocytes and peripheral blood leucocytes (PBL), although cytotoxicity was not generated in such cultures. After day 14, non-specific cytotoxicity was exhibited by both lymphocyte populations and was maximal in moribund calves. Non-specific lytic activity was also noted in PBL from recovering calves, but disappeared after the elimination of the parasite. Calves immunized against T. parva by the concurrent administration stabilate and long-acting tetracycline showed macroschizont forms of the parasite in host lymphocytes from days 15 to 18 after infection. During this period, lymph node cells from infected calves stimulated MLR-type responses in autologous PBL. However, PBL from immunized calves lysed directly, autologous infected lymphoblasts in a genetically-restricted fashion from days 14 to 21, although additional cytotoxicity was not generated in the MLR. The results imply that when the parasitosis of T. parva is curtailed, specific cell-mediated responses are mounted against parasite-induced antigens in combination with polymorphic host antigens on the leucocyte membrane. These reactions are probably the major immune responses conferring to recovered cattle, immunity against rechallenge with the homologous isolate of T. parva.