This study, which is part of a larger immunohistochemical investigation of blood-group antigens in non-neoplastic urothelium and bladder cancer, reports our findings on the expression of an antigen related to the cryptic Thomsen-Friedenreich antigen (beta Gal 1-3 GalNAc) of erythrocytes. De-waxed sections of 19 ureters and of 93 transitional cell tumours, either untreated or pretreated with neuraminidase, were subjected to an indirect immunoperoxidase staining, employing the monoclonal antibody 49H.8. Staining results were compared to Lewis-secretor types, morphology, and in tumours to the clinical course as regards recurrence rate and the development of either stroma invasive recurrence or papillomatosis as well. Ureters not subjected to neuraminidase were unstained, whereas urothelium in 12 of 19 ureters subjected to neuraminidase showed staining. Serial dilution of antibody disclosed quantitative differences related to the Lewis-secretor types. Lea+b- urothelium, i.e., non-secretor urothelium, had the highest end-point titers. Endothelium was unstained. Thirty-six of the 93 tumours showed staining without prior neuraminidase treatment, 31 showed staining after neuraminidase treatment only, while 26 were unstained. Staining correlated with the pathological stage and grade (p less than 0.05), but not with the clinical course (p greater than 0.05). The results do not support previous observations on the prognostic value of Thomsen-Friedenreich antigen determination in superficial bladder cancer.