Cross-resistance to anticancer drugs, termed multidrug resistance (mdr), has been functionally associated with the expression of a plasma membrane energy-dependent efflux pump, termed P-glycoprotein, the product of the mdr1 gene. When MCF-7 breast carcinoma cells were transfected with the human mdr1 gene (BC-19 cells), they expressed levels of P-glycoprotein equivalent to those of cells selected for resistance to doxorubicin (MCF-7/ADR) but exhibited 10- to 50-fold less resistance to doxorubicin and vinblastine. We have now demonstrated that when BC-19 cells were stably transfected with protein kinase C alpha (PKC alpha), resistance to doxorubicin and vinblastine was increased; wild-type MCF-7 cells transfected with PKC alpha did not exhibit any change in drug resistance. Increased resistance in PKC alpha-transfected BC-19 cells was associated with enhanced PKC activity and phosphorylation of P-glycoprotein and decreased drug accumulation. The PKC activator, phorbol dibutyrate, further increased resistance to doxorubicin and stimulated P-glycoprotein phosphorylation. These results demonstrate that transfection of P-glycoprotein-expressing cells with PKC resulted in increased mdr and that PKC may have served as an important modulator of this process.