Immunoperoxidase localization of A and B blood group substances and carcinoembryonic antigen in papillary transitional cell carcinoma of the bladder demonstrated blood group substances in 22 of 48 carcinomas and trace amounts of carcinoembryonic antigen in 26. All 22 cases of transitional cell carcinoma positive for blood group substances were stage O (noninvasive) and 21 were grade I or II (well differentiated). Of 26 tumors negative for blood group substances 14 were grade III (poorly differentiated). Of 17 patients who had tumors positive for blood group substances at any time no positive tumor was followed by invasive carcinoma. In contrast, of 17 patients who had tumors negative for blood group substances 15 either had invasive carcinoma at the time of diagnosis or later. Carcinoembryonic antigen positivity did not correlate with pathologic stage, histologic grade or clinical course.