We used monoclonal antibody 486P 3/12 to monitor 55 patients after transurethral resection of stages Ta and T1 transitional cell bladder carcinoma. The method has a high sensitivity (89%) in detecting grades 1 to 3 lesions. We examined voided urinary specimens taken every 4 weeks after transurethral resection for a mean of approximately 2 years. Of the 55 patients 22 remained negative for marker without prophylaxis and only 2 of them had another tumor. Only 6 of 33 patients who were positive for marker after transurethral resection were negative for marker at least once immediately after transurethral resection. Of the 33 patients 14 had a recurrent tumor. In all 14 patients a marker-positive urine specimen preceded visible tumor recurrence by several months. Monoclonal antibody 486P 3/12 is a safe marker to identify patients with superficial bladder carcinoma after transurethral resection who do not require prophylaxis. The 22 patients who remained negative for marker after transurethral resection had an almost equal distribution of stages Ta to T1 lesions (9 versus 13) and equal distribution of grades 1 and 2 lesions (11 versus 11).