A and B blood group specific monoclonal antibodies have been used in an indirect immunoperoxidase technique to detect blood group isoantigen (BGI) on normal transitional epithelium and its possible loss on malignant cells. On all 14 specimens of normal tissue and on 31 of 39 (79%) superficial transitional cell carcinomas of the bladder BGI was detected. The incidence of invasive recurrence in the BGI-negative group (50%) was significantly higher than in the BGI-positive group (13%). It is concluded that this method of detecting BGI expression offers several advantages over existing techniques and identifies a group of patients with superficial transitional cell carcinomas of the bladder who are at higher risk of developing an invasive recurrence.