Cyclosporin A abrogates pleiotropic drug resistance in certain experimental tumors. Its impact on drug-sensitive tumors has not been investigated. Our studies show that in drug-sensitive Ehrlich ascites carcinoma and hepatoma 129 cyclosporin A enhances daunorubicin inhibition of DNA synthesis in vitro and prolongs survival of host mice in vivo. Of particular interest is that cyclosporin A converts ineffective daunorubicin regimens into those which result in prolongation of host mice survival. Other agents known to reverse pleiotropic drug resistance are reported to exert their effects by increasing intracellular drug accumulation. In contrast, our studies of drug transport in drug-sensitive Ehrlich ascites carcinoma and hepatoma 129 show that cyclosporin A causes minimal enhancement of [3H]daunorubicin uptake without inhibition of [3H]daunorubicin efflux in both the presence and absence of interrupted active daunorubicin efflux. This suggests that the mechanism of action of daunorubicin enhancement by cyclosporin A in drug-sensitive tumors is not simply the result of increased intracellular daunorubicin accumulation. In vivo dosages of cyclosporin A in the current study are comparable to those which can be used with reasonable safety in humans. We conclude that cyclosporin A may be useful in the potentiation of anthracycline antibiotic therapy directed against drug-sensitive as well as drug-resistant tumors.