Anthracyclines such as Adriamycin (ADR) and daunomycin markedly inhibit cell growth in vivo and in vitro. These studies demonstrate that 30 microM hemin, which induces hemoglobin synthesis in human and murine erythroleukemia cells in culture, markedly decreases the cytotoxicity of ADR in a variety of hemopoietic cell lines (K562, HEL-1, MEL-745, HL-60, and U937) and in erythroid burst-forming cells from normal human marrow. Hemin failed to protect four of the five nonhemopoietic cell lines tested, including MCF-, breast adenocarcinoma cells, C-205 colon carcinoma cells, mouse 3T3 fibroblasts, and mouse kidney VERO cells. Hemin did protect human neuroblastoma IMP-32 cells from ADR cytotoxicity; however, this nonhemopoietic cell line undergoes dendrite formation in response to hemin induction. Cytofluorographic analysis of cellular ADR content and labeling studies with [3H]daunomycin demonstrated that hemin decreases the intracellular accumulation of these anthracyclines by more than 50% in K562 erythroleukemia cells. These studies indicate that small doses of hemin prevent intracellular accumulation of anthracyclines and thereby markedly reduce anthracycline toxicity to cells. Since this protective effect is observed preferentially with hemopoietic cells, it is possible that this finding could be exploited to protect the bone marrow from the cytotoxic action of anthracyclines during therapy for nonhemopoietic tumors.