Infection and transformation of cells of the bovine immune system by Theileria annulata and T. parva were compared. Preliminary experiments with mammary gland macrophages indicated that they were permissive to infection by T. annulata but only to a limited extent by T. parva. Further experiments involved several purified subpopulations of bovine cells including bovine monocytes, T cells and MHC class II positive and negative populations. These subpopulations were incubated with T. annulata or T. parva sporozoites in limiting dilution cultures. T. annulata preferentially infected macrophage type cells and also MHC class II positive cells, whereas the frequency of MHC class II negative cells infected by this parasite was negligible. T cells also showed a very low level of infection. In complete contrast, T. parva preferentially infected T cells and did not infect cells phenotypically defined as monocytes at all. These results suggested that class II expression was necessary for T. annulata infection and not necessary for, though not a barrier to T. parva infection. T. annulata infected cell lines all expressed class II molecules to varying degrees. Other available phenotypic markers were only expressed at very low levels or no longer expressed. The immunological significance of the different cell preferences and phenotypes of infected cell lines of T. annulata and T. parva is discussed.