A direct comparison of the intracellular pharmacokinetics of 4'-epi-doxorubicin and doxorubicin was carried out in five patients with leukemia who were given weekly low doses of a combination of these drugs at 20 mg each in an i.v. injection. Blood samples were collected for 48 h after administration and the drug concentrations in leukemic cells were determined by high-performance liquid chromatography (HPLC). The intracellular peak concentrations of 4'-epi-doxorubicin were higher than those of doxorubicin in all patients. The AUC for the intracellular drug concentration vs time curve was significantly higher for 4'-epi-doxorubicin. The intracellular uptake and retention were also studied in vitro after incubation of isolated leukemic blast cells with the two drugs; they showed the same pattern observed in vivo. We conclude that 4'-epi-doxorubicin and doxorubicin exhibit different pharmacokinetics in malignant cells. The therapeutic significance of this finding requires further evaluation.