Cremophor EL (polyoxyethylene castor oil) and Tween 80, used as solvents for cyclosporin A and VP-16, respectively, were found to reverse the multidrug resistant (MDR) phenotype. In daunorubicin (DNR) resistant Ehrlich ascites tumor cells (EHR2/DNR+), both solvents at percentages of 0.01% (v/v) enhanced DNR accumulation to sensitive levels. Cremophor EL and Tween 80 did not influence DNR accumulation in drug-sensitive cells (EHR2). The concentration of cyclosporin A alone that enhanced DNR accumulation in EHR2/DNR+ cells to sensitive levels was 5 micrograms/mL whereas 0.2 micrograms/mL of cyclosporin A dissolved in 0.001% (v/v) Cremophor EL enhanced DNR accumulation to sensitive levels, thus indicating synergy between Cremophor EL and cyclosporin A. Cyclosporin A had a negligible effect on DNR accumulation in the drug-sensitive cells. In clonogenic assays, the LD10 of DNR was 1 microM in EHR2/DNR+ cells. Combining 1 microM DNR with non-toxic amounts of Cremophor EL (0.001% and 0.002%, v/v) potentiated the cytotoxicity of DNR and resulted in a cell kill of 77% and 86%, respectively, in the resistant cells. In non-toxic amounts, CrEL and Tween 80 acted synergistically with reduced concentrations of verapamil, resulting in DNR accumulation approaching close to the sensitive level. Azidopine photoaffinity labeling of P-glycoprotein in plasma membrane vesicles from EHR2/DNR+ cells was inhibited 100% and 80%, by 0.003% (v/v) Cremophor EL or Tween 80, respectively. These data permit the conclusion that non-toxic amounts of CrEL and Tween 80 modulated DNR resistance by raising intracellular DNR levels, due to their abilities to bind to the plasma membrane P-glycoprotein.