Bleomycin, in concentrations similar to those present after drug administration, was a potent inhibitor in vitro of both human phytohemagglutinin (PHA)-stimulated and pokeweed mitogen (PMW)-stimulated lymphocyte transformation, but lower concentrations were much more inhibitory to the former. Bleomycin showed minimal or no in vitro myelosuppression. The similarity of these in vitro results to in vivo results by others suggests the in vitro system used here may be useful to screen new drugs for potential in vivo suppression of either the myeloid or the lymphoid tissue. The data suggest that by adjusting the dose of bleomycin inhibition of lymphocytes stimulated by PHA with only minimal inhibition of lymphocytes stimulated by PWM may be possible. Variable response of lymphoid neoplasia to bleomycin may in part reflect whether the cell type is T or B in origin, in view of work by others suggesting that PHA is more stimulatory to T cells and PWM is stimulatory to both B and T cells.