Epirubicin, at concentrations ranging between 10(-7) and 10(-13) M, was assayed in semisolid cultures of human normal hemopoietic cells and in liquid cultures of 5 different human leukemic cell lines. The growth of all normal hemopoietic progenitor cells was inhibited by the higher drug concentrations; at the lowest concentration, only CFU-E and 7th-day CFU-GM were not inhibited. On the other hand, leukemic cells were sensible only to the higher concentration of epirubicin, which, nevertheless, was not fully inhibitory. Leukemic cell differentiation was not promoted by the drug, as evidenced by a panel of monoclonal antibodies, by cytochemistry and by functional tests. These results suggest a marked in vitro myelotoxicity of epirubicin, that does not appear to be compensated by a powerful control of leukemic cell proliferation.