We used monoclonal antibody 486P 3/12, anti-ABO(H) antibodies and anti-T antigen lectins to detect malignant transformation in cystectomy specimens from patients with stage T2 transitional cell carcinoma of the bladder. We used an immunoperoxidase technique to do extensive chessboard-like mapping studies in 6 cystectomy bladders with stage T2 lesions and 1 normal bladder from a multiorgan donor as control to characterize antigen expression. Increased 486P 3/12 antigen expression, decreased ABO(H) expression and T antigen deletion were detectable not only in the tumor area but also in tumor-surrounding areas classified as benign. We believe that cells with abnormal antigen expression are a source of tumor recurrence and can be identified with our method of quantitative immunocytology.