The diagnosis of transitional cell carcinoma by cytological examination of exfoliated urinary cells is important in the early detection and followup of patients with this disease. Proper interpretation requires a skilled pathologist. Accuracy also is influenced by collection methods and nonmalignant pathological conditions of the bladder. An immunocytochemical technique using monoclonal antibodies G4 and E7 successfully identified tumor-associated antigens on the surface of transitional carcinoma cells obtained by bladder washings. The method, which uses immunoperoxidase staining, was compared to conventional Papanicolaou staining of bladder washings from 75 patients with and without transitional cell carcinoma. Patients were divided into 4 groups: group 1 (nontumor control)--15 patients with no pathological condition of the bladder or nonmalignant urological diseases, group 2 (nontransitional cell carcinoma)--19 patients with other urological malignancies, group 3-18 patients with active transitional cell carcinoma and group 4-23 patients with a history of transitional cell carcinoma but no evidence of tumor at the time of the washing. The incidence of positive staining in these groups was 0, 5, 78 and 0 per cent, respectively. The diagnostic value of immunoperoxidase staining was similar to that of Papanicolaou staining in the control group and in patients with high grade transitional cell carcinoma, and provided specific morphological criteria not possible by conventional cytology studies. Interpretation of immunoperoxidase staining was difficult in washings with a large number of inflammatory cells if endogenous peroxidase activity was not blocked properly. The application of the immunoperoxidase staining method for diagnosis of low grade tumor is under further investigation.