Employing the indirect immunoperoxidase technique, monoclonal antisera against blood group antigens A and B were used to localize the corresponding tissue isoantigens in normal ureter and transitional cell carcinoma of the ureter and renal pelvis in 29 patients. All five cases of normal ureters showed positive staining of tissue isoantigens within the transitional epithelium, and all twelve cases of noninvasive transitional cell carcinoma showed similar staining in tumor cells. Of the remaining twelve cases who had invasive tumor, eight lacked tissue isoantigens, while four cases exhibited positive staining. These results support the earlier findings that normal urothelium and noninvasive transitional cell carcinoma of the urinary tract possess ABO tissue isoantigens, while these isoantigens are most frequently absent in invasive tumors. In addition, this study also demonstrates that invasive transitional cell carcinoma of ureters and renal pelvis may continue to possess tissue isoantigens when studied by this sensitive, specific method.