The expression of a cell surface antigen recognized by the epithelium-specific, tumor-associated monoclonal antibody (Moab) AUA1 has been studied in 22 cases of normal urothelium, 7 cases of dysplastic urothelium and 86 cases of transitional cell carcinoma. Tissue specimens were stained with the conventional hematoxylin-eosin technique as well as with the indirect immunoperoxidase technique using Moab AUA1. Results showed that: (a) Moab AUA1 reacts mainly with cells of high or intermediate grade transitional cell carcinoma and with a restricted number of normal urothelial cells, and (b) antigenic heterogeneity exists between tumors of the same grade and within the same tumor.