Partially purified extracts from human urothelial tumors were utilized as competing antigens in competition radioimmunoassays in conjunction with purified RNA viral interspecies proteins and the respective antibodies to these viral proteins in efforts to detect the presence of one of the structural components of type C RNA viruses, the p30 core protein. Some antigen present in extracts of 25% of the bladder and 22% of the prostate tissues assayed demonstrated cross-reactivity with the viral p30 protein used in the radioimmunoassay system. These findings suggest that some human urothelial tissues contain a protein similar to the p30 core protein of the C-type RNA viruses and that this protein might prove useful in clinical surveys of patients with urogenital tumors.