Staurosporine, a potent inhibitor of C-kinase, enhances accumulation of vincristine (VCR) in multidrug-resistant cells. We investigated this enhancement by two methods: (I) ATP-dependent VCR binding system; (II) azidopine photolabeling system. The ATP-dependent VCR binding to the resistant cell membrane was inhibited more efficiently by staurosporine than by verapamil. Staurosporine also inhibited the azidopine photolabeling of P-glycoprotein. These results indicate that staurosporine, an inhibitor of C-kinase, might directly bind to P-glycoprotein as well as antitumor agents and Ca2+ channel blockers. These findings also indicate that C-kinase might be involved in the function of P-glycoprotein.