Four different cytostatic compounds (prospidium chloride, peptichemio, dacarbazine and mithramycin) have been assayed for their effect on lymphoblastic transformation of spleen cells from mice. The drugs did not affect cell viability at concentrations lower than 0.1–0.2 μg/ml (peptichemio and mithramycin) or than 10–20 μg/ml (prospidium chloride and dacarbazine). Except for peptichemio, which did not show any marked effect, these cytostatics acted more actively on B cells than on T cells at concentrations of drug not affecting lymphocyte viability. Mithramycin was the most active inhibitor of the mitogen-induced DNA synthesis in the cell. Concentrations of this drug to inhibit the blastogenic response to 50% (IC50) were lower than 0.1 μg/ml. Inhibition of mitogenesis was less pronounced in the case of dacarbazine (IC50 = 50 and 10 μg/ml for T and B cells, respectively), prospidium chloride (IC50 > 50 and 50 μg/ml for T and B cells) and peptichemio (IC50 = 0.25 and 0.5 μg/ml for T and B cells, respectively).