Cell lines infected with Theileria parva were derived by infection of bovine peripheral blood lymphocytes with sporozoites in vitro. Cattle were inoculated with doses of autologous infected cells ranging from 1 × 10 1 to 1 × 10 8. Infection became established in animals which received 1 × 10 2 or more cells. While 1 × 10 2 cells resulted in sub-patent infection with development of immunity to challenge with sporozoites, larger doses of cells gave rise to patent infections of increasing severity. Thus, doses of 1 × 10 5 and 1 × 10 6 cells sometimes produced lethal infections and with 1 × 10 7 and 1 × 10 8 the outcome was invariably lethal. Based on the previous observation that induction of immunity by allogeneic cells requires transfer of infection into the recipient-host cells, a comparison of the infections produced by autologous and allogeneic cells indicated that the transfer of infection from allogeneic cells occurs at a frequency of maximally 1 × 10 −5. Two pairs of cattle were identified as being mutually non-reactive in the mixed leukocyte reaction (MLR). Doses of 1 × 10 6 and 1 × 10 7 cells of cell lines derived from 1 animal of each pair were inoculated into the autologous host, the non-reactive partner and an animal which was shown to be strongly reactive to the donor in the MLR. In each instance, the reaction in the MLR non-reactive recipient was not significantly different from that of the MLR reactive recipient, but was markedly different from that of the autologous recipient.