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The target cells for infection and transformation by Theileria parva were investigated. Peripheral blood mononuclear cells were reacted with monoclonal antibodies specific for bovine leukocyte differentiation antigens, sorted into subpopulations with a fluorescence-activated cell sorter, and infected in vitro with T. parva sporozoites. Infected cells were cultured at limiting dilution, and transformed clones were screened with monoclonal antibodies. The results indicated that B cells, T cells (including BoT4+ and BoT8+ cells), and null cells but not monocytes or neutrophils were transformed in vitro after infection with T. parva. After transformation, peripheral blood T cells and T-cell clones retained expression of most or all of the T-cell differentiation antigens including the mature T-cell marker recognized by monoclonal antibody IL-A27, BoT2, and BoT4 or BoT8, and some cells acquired a low level of expression of BoT4, BoT8, or the null cell marker recognized by monoclonal antibody IL-A29. T. parva-transformed null cells retained expression of the IL-A29 determinant and acquired expression of BoT2 and BoT8 but not the IL-A27 determinant or BoT4. T. parva-transformed B cells in most instances lost expression of surface immunoglobulin and never acquired expression of the IL-A27 determinant, BoT2, BoT4, or BoT8, although some cells acquired a low level of expression of the null cell marker recognized by monoclonal antibody IL-A29. Further studies on cell lines and clones grown in vitro from populations isolated from T. parva-infected cattle suggested that the majority of the in vivo T. parva-transformed cells were of T-cell origin.