Expression of the multidrug-resistance gene product P-glycoprotein (P-170) was screened in 21 untreated human renal cell carcinomas using monoclonal antibodies (265/F4, C219) and immunoperoxidase staining. The inherent drug resistance of the same samples against doxorubicin was established by a short-term chemoresistance test in order to investigate the association between the expression of P-170 and intrinsic drug resistance in kidney cancers. P-glycoprotein could be demonstrated in 10 cases. Using the short-term test for predicting resistance, 17 resistant and 4 sensitive cancers were found. In vitro 10 of 17 resistant tumors revealed an increase of P-glycoprotein. On the other hand, in the sensitive tumor in vitro, an expression of P-glycoprotein could not be demonstrated. This investigation reveals that intrinsic drug resistance exists in many renal cell carcinomas and it is associated at least in part with increased expression of P-glycoprotein. The immunohistochemical results suggest that the presence of the P-glycoprotein may be useful as a marker for screening the multidrug-resistant phenotype in renal cell carcinomas and as an indicator of the therapeutic efficacy of multidrug-resistant kidney cancers.