4-Demethoxydaunorubicin (idarubicin, IDA) is an anthracycline that has shown good cytotoxic activity in vitro against tumor cell lines displaying the multidrug-resistant (MDR) phenotype. IDA is converted in the liver into idarubicinol (2HIDA) and, in this form, seems to exert its antitumoral activity in vivo. Recent studies have shown that 2HIDA has tumoricidal activity similar to that of the parent drug when tested in vitro in sensitive neoplastic cells. In this work we compared in vitro the effects of IDA and 2HIDA used alone and in combination with 2 microM cyclosporin A (CyA) in the MDR leukemic cell lines FLCR and K562R and in their sensitive parent cell lines FLC and K562. IDA and 2HIDA showed the same cytotoxic activity in sensitive cells. After 1 h of exposure of cells to each anthracycline, we observed that the cellular uptake of IDA and 2HIDA was also similar. In resistant cells, 2HIDA was 3-4 times less active than IDA. We observed that the intracellular uptake of 2HIDA was lower than that of IDA, and this may be correlated with a greater ability of P-glycoprotein to expel 2HIDA as opposed to IDA. Indeed, when MDR cells were exposed to IDA and 2HIDA in combination with 2 microM CyA, the cytotoxic effect of these anthracyclines was the same, and it was similar to that observed in sensitive cells. These data confirm the utility of the combination of IDA and an MDR-reversing agent in hematological malignancies displaying the MDR phenotype.