The intracellular uptake, retention and cytotoxicity of adriamycin (ADR) combined with a biscoclaulin alkaloid, cepharanthine, were investigated by flow cytometry in NIH 3T3 cells. Cepharanthine suppressed the efflux of ADR in a similar fashion to verapamil. The intracellular uptake and retention of ADR were increased gradually by 0.1 to 1 microgram/ml of cepharanthine and reached a plateau at greater than 1 microgram/ml. Cepharanthine, which had no toxic action on survival, increased intracellular ADR uptake by about 20% for 1 h incubation at 37 degrees C, and increased cellular ADR retention after incubation in an ADR-free medium for 4 h from 15% to 75%. The cytotoxicity of ADR was enhanced 5-fold in the cells pre- and co-incubated with cepharanthine. When cepharanthine was present in the medium before, during and for colony formation (10 days) after incubation with ADR, the cytotoxicity increased to about 300-fold. Furthermore, an increase in intracellular uptake of ADR was induced by an elevated temperature of 43 degrees C, and the efflux of ADR was inhibited by cepharanthine. A high level of intracellular ADR was maintained during the treatment. These results suggest a possible novel use of cepharanthine to improve the drug sensitivity of tumors resistant to ADR.