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Transitional carcinomas of the urinary bladder were examined immunohistochemically for keratin proteins with the use of polyclonal antiserum (TK, 41-65 kDa) and 3 monoclonal antibodies (KL 1, 55-57 kDa; PKK 1, nos. 19, 18, 8; and K 8.12, nos. 16, 13). Umbrella cells gave particularly strong staining for TK, KL 1 and PKK 1, whereas they were negative for K 8.12. Basal- and intermediate-layer cells in urothelial epithelium were moderately positive for all keratins. Brunn's nests cells showed comparatively slight or moderate keratin staining, and K 8.12 staining of Brunn's nests was higher than in urothelial epithelial cells. Transitional carcinoma (grades I and II) indicated uniform keratin distribution, and staining was strong with TK, while that of KL 1, PKK 1 and K 8.12 varied, and grade III tumors showed the lowest intensity of staining. K 8.12 staining in papillary transitional carcinomas was strongly positive in basal located tumor cells, as compared with apical tumor cells. Squamous cell carcinoma was varying positive to keratin reactions dependent on the degree of keratinization. Heterogenity of keratin distribution in papillary transitional carcinomas was given between basal tumor cells and well differentiated tumor cells including umbrella-like cells.