The participation of genetically restricted cell-mediated lympholysis (CML) in protective immunity against Theileria parva was examined in cattle inoculated with autologous and allogeneic parasitized lymphoblasts. The majority of cattle given 10(7) infected allogeneic cells were protected against infection with the homologous parasite. Animals inoculated with drug-cured lymphoblasts, normal allogeneic lymph node cells (LNC), or a combination of LNC and parasitized allogeneic cells were fully susceptible. The magnitude of the primary CML responses against autologous infected targets (as opposed to that against the allogeneic lymphoblasts inoculated or 3rd-party targets) more accurately correlated with a decreased severity of infection after challenge. Autologous CML responses appeared earlier as cattle were given increasing numbers of parasitized autologous lymphoblasts, and were higher and more persistent with increasing parasitosis. Between 10(2) and 10(5) autologous infected cells induced immunity to T. parva, whereas at least 10(7) allogeneic lymphoblasts were required for comparable protection. The findings provide additional evidence for a protective role of restricted CML responses against T. parva in vivo. When allogeneic infected cells are used for immunization, viable T. parva need to establish infection in host cells to induce adequate protective immunity.