Leukemic cells from patients presenting with acute nonlymphoblastic leukemia and normal hematopoietic bone marrow cells from healthy donors for allogeneic bone marrow transplantation were incubated for 3 h with doxorubicin and epirubicin at different concentrations. The intracellular uptake at the end of the incubation was determined by photofluorometry in leukemic cells from 15 patients and in normal cells from 9 donors for bone marrow transplantation. Cytotoxicity in vitro against granulocyte/macrophage colony-forming units (CFU-GM) was determined in normal cells from 7 donors, and in vitro toxicity against leukemic cells was determined by a clonogenic technique in cells from 6 patients and by vital dye staining (DiSC) following 4 days' culture in cells from 15 patients. Epirubicin was significantly less toxic than doxorubicin to normal hematopoetic cells (72% +/- 20% survival of cells for epirubicin vs 45% +/- 13% for doxorubicin at a concentration of 0.2 microM; P less than or equal to 0.005). As analyzed by the DiSC assay, 0.2 microM epirubicin was slightly more toxic to leukemic cells than was the same concentration of doxorubicin (47% vs 61% survival, P less than or equal to 0.01), but the clonogenic assay revealed no difference in toxicity to leukemic cells. At a concentration of 0.2 microM, the mean intracellular uptake of epirubicin in leukemic cells was 0.43 +/- 0.26 nmol/mg protein as compared with 0.33 +/- 0.14 nmol/mg protein for doxorubicin (not significant). In normal cells, the uptake of epirubicin at a concentration of 0.2 microM was 0.47 +/- 0.25 nmol/mg protein as compared with 0.31 +/- 0.21 nmol/mg protein for doxorubicin (not significant). The reduced myelotoxicity observed in vitro together with the retained toxicity to leukemic cells indicates that the therapeutic index of epirubicin is better than that of doxorubicin.