Combretastatin A4, a novel anti-mitotic agent was effective against two P388 cell lines with acquired resistance to daunorubicin. In contrast, Combretastatin A1, a close structural analogue of A4, showed a high degree of cross-resistance. Combretastatin A1 was also more efficient at increasing intracellular daunorubicin concentrations in both resistant cell lines. Neither agent was capable of altering anthracycline accumulation in the parental (sensitive) cell line. We propose that the cross-resistance to Combretastatin A1 occurs, at least in part, as a result of the increased affinity of the drug-efflux process operative in these resistant cells for Combretastatin A1 vs Combretastatin A4. Hence, Combretastatin A4 may play a role in the treatment of tumours with acquired resistance to the anthracycline antibiotics.