Summary—The indirect immunoperoxidase test to detect urothelial cell surface blood group antigens was performed in an attempt to assess the loss of these antigens as a prognostic predictor. In our technique A, B and O(H) blood group specific monoclonal antibodies were used. It was possible to perform the test satisfactorily on histological material from 55 cases which were superficial on presentation; 15 subsequently became invasive. The loss of surface antigens correlated well with histological grade—44.4% of G1, 87.2% of G2 and 94.1% of G3 tumours were antigen‐negative.The blood group phenotype of 114 patients was studied. In the group that subsequently became invasive, 14 of 26 (54%) were of blood group A phenotype compared with 37 of 88 (42%) in the non‐invasive category.Acetylation studies were performed on 47 cases. In the group that subsequently became invasive, 5 of 10 (50%) had the slow phenotype of the enzyme N‐acetyltransferase compared with 13 of 37 (35%) in the group that remained su