We studied whether inducers of cell differentiation alone could have cytotoxic effect on the promonocytic U937 and Mono Mac 6 cells in vitro. The cells were incubated with standard differentiating doses of interferon (IFN)-gamma, dibutyryl cAMP (Bt2cAMP) or the phorbol ester phorbol-12-myristate-13-acetate (PMA), with or without lipopolysaccharide (LPS), and both protein synthesis and viability were examined. In both U937 and Mono Mac 6 cells the incorporation of [3H]leucine was significantly reduced after PMA plus LPS stimulation, but not after IFN-gamma stimulation, when compared with controls. For U937 cells there was also reduced incorporation after Bt2cAMP stimulation. Trypan blue exclusion experiments and the number of cells remaining in the cultures indicated that Bt2cAMP-, PMA- and/or LPS-stimulated, but not IFN-gamma-stimulated, cells were less viable than unstimulated U937 or Mono Mac 6 cells. The results suggest that Bt2cAMP, PMA and LPS, but not IFN-gamma, are cytotoxic towards promonocytic cancer cell lines in vitro.