A Theileria parva lawrencei isolate in the form of a sporozoite stabilate, derived by feeding clean Rhipicephalus appendiculatus nymphal ticks on an African buffalo (Syncerus caffer) captured in the Laikipia District, Kenya, was inoculated into groups of cattle at dilutions between 100 and 10-3. Groups of 3 cattle infected with 1 ml inocula at 100, 10-1 and 10-2 dilutions were treated with 2·5 mg/kg body weight of buparvaquone on day 0 and similar groups were left untreated to act as controls. An additional group, given 100 dilution of the stabilate, was treated with buparvaquone on day 8 post-inoculation. It was found that all control cattle inoculated with the stabilate at dilutions between 100 and 10-2 became infected, but only 2 out of 3 cattle developed patent infections at 10-3 dilution. All 3 control cattle receiving 100 dilution died of theileriosis, 2 at 10-1 and 10-2 dilutions, and 1 at 10-3 dilution died. Buparvaquone treatment on day 0 at 100 dilution resulted in the survival of 2 of 3 cattle and all the cattle at 10-1 and 10-2 dilutions. All the surviving cattle eventually developed a significant serological response against T. parva in the indirect fluorescent antibody test, except 1 in the 10-3 dilution group, and were immune to homologous challenge when tested 3 months later with a lethal inoculum of stabilate, except 2 cattle in the 10-3 dilution group. As a result of a theileriosis problem at about day 60 after inoculation in 2 cattle given 10-2 dilution of stabilate and buparvaquone treatment on day 0, an additional 5 cattle were given 10-2 dilution of stabilate and developed a good immunity after buparvaquone treatment. None was shown to develop the carrier state. Treatment with buparvaquone on day 8 after infection with 100 dilution of stabilate was not successful since 2 died. The stabilate used was shown to produce reproducible infection in cattle at different dilutions.