Expression of blood group antigens A, B, and H was studied in transitional carcinoma of the urinary bladder. Positive result was obtained in 16 cases of a group of 21 tumours by indirect immunoperoxidase reaction with monoclonal antibodies. A relation of quantitative expression to histological grading of tumour differentiation was noticed. Well differentiated transitional carcinomas mostly had more conspicuous expression of A, B, and H antigens. However, expected more favourable prognosis in cases of transitional carcinoma with expression of A, B, and H blood group antigens failed to be proved.