Reversal of multidrug drug resistance (MDR) has been achieved in vitro by a variety of agents including verapamil, quinidine, cyclosporine A, and amiodarone. The toxicity of these agents precludes the achievement of sufficient levels in the serum to circumvent efficiently the MDR in vivo. The authors previously demonstrated that quinine, the widely used antimalarial agent, is able to reverse primary resistance of rat colon cancer cells to anthracyclines. In this report, the efficiency of quinine formiate in reversing the doxorubicin (ADM) (Adriamycin, Adria Laboratories, Columbus, OH) resistance of the well-defined MDR human leukemic cell line K562/ADM was demonstrated. In culture medium, quinine is slightly less effective than verapamil in increasing the cytotoxicity and uptake of ADM when both drugs are used at the same concentration. A nontoxic dose of 5 micrograms/ml is necessary to reverse the MDR in K562/ADM cells. In patients receiving quinine formiate in a continuous intravenous infusion, a significant correlation (r = 0.84) was found between the serum levels of quinine and the ability of sera to increase ADM uptake in K562/ADM cells. When quinine is administered at a conventional dose (25 to 30 mg/kg/d), serum levels consistently reach more than 8 micrograms/ml without severe side effects; ear noises and vertigo are the dose-limiting side effects. At these concentrations, quinine induces a more than double increase in ADM uptake in K562/ADM cells. Pharmacokinetic data indicate that quinine should be administered 24 to 36 hours before anti-cancer drugs in clinical trials that test its efficiency as a modifier of MDR in human hematologic malignant neoplasms.