A monoclonal antibody, MRK16, recognizing specifically an epitope of P-glycoprotein (P-GP) was used to determine the degree of expression of P-GP in kidney and urinary bladder cancers. Immunohistochemistry, immunoelectronmicroscopy, and immunoprecipitation were used for this study. Expression of P-GP was found in 6 of 20 kidney cancers treated without anticancer drugs. Also expression of P-GP was found in 17 of 47 urinary bladder cancers. 11 of 31 in primary cases, 0 of 5 in recurrent cases treated without anticancer drugs, and 6 of 11 in recurrent cases treated with anticancer drugs were positively expressed. These results indicate that a certain proportion of kidney and urinary bladder cancers intrinsically acquired multidrug resistance, and also that prior administration of anticancer drugs may induce P-GP in initially negative tumors. We also succeeded to detect MDR1 mRNA by means of in situ hybridization. From our present data, our methods to detect P-GP and MDR1 mRNA appeared to be very useful from the point of clinical application.