The antigenic expression of normal bladder epithelium and transitional carcinomas has been studied using the epithelium-specific, tumour-associated monoclonal antibodies HMFG1 and AUA1. Tissues from 79 cases of bladder carcinoma and 11 cases of non-neoplastic bladder tissues were stained with both the haematoxylin-eosin (H/E) and the indirect two-stage immunoperoxidase methods using the monoclonal antibodies (MAbs) HMFG1 and AUA1 at a concentration of 25 micrograms ml-1. Positive and negative controls were also used. Moreover, 46 urine smears prepared after cytocentrifugation were stained with both the Papanicolaou and the indirect two-stage immunoperoxidase methods. The results showed that HMFG1 reacted with the majority of cases of grade III carcinomas and carcinomas in situ and with a subset only of low-grade (I and II) carcinomas. The pattern of staining showed the following characteristics: (1) the epithelial surface membrane stained both in normal bladder and bladder carcinomas (surface of the papillae), (2) a variant number of cancer cells, increasing with the degree of malignancy, showed membrane and/or cytoplasmic staining, (3) tumours of the same histological grade showed antigenic heterogeneity. The MAb AUA1 was not widely expressed. The immunocytochemical study confirmed the reaction of HMFG1 with a variant number of malignant urothelial cells exfoliated in urine. Their reaction with AUA1 was much more limited. The immunocytochemical staining seemed to be more sensitive in the detection of malignant cells in some cases which had been characterized as negative or suspicious for malignancy by the Papanicolaou examination. The intravesical treatment with chemotherapeutic agents did not seem to influence the antigenic expression of malignant urothelial cells.