Monoclonal antibodies directed against tumor-associated antigens of bladder carcinoma were used to identify tumor cells in bladder washout specimens of 40 patients with bladder carcinoma (group 1), 41 with no bladder disease or with urinary tract infections (group 2), 41 who received long-term mitomycin C instillation therapy after excision of the tumors (group 3) and 39 who received no prophylaxis after excision of the tumors (group 4). In all groups the same bladder washout specimen was used for standard urinary cytological and immunocytological tests. True positive results were obtained in 90 per cent of the patients in group 1 according to our immunocytological criteria and in 43 per cent according to standard cytology studies. No urine specimens in group 2 (controls) were immunocytologically positive, while 16 of 41 in group 3 and 17 of 39 in group 4 were positive immunocytologically but only 4 and 5, respectively, were positive according to standard cytology studies. Further followup of these patients will show whether cells positive for monoclonal antibody 486 P 3/12 will permit early detection of recurrent bladder cancer and whether one can identify patients who require prophylaxis after removal of the superficial bladder tumors.