Recent evidence indicates that the Lewis X determinant is a tumor-associated antigen in the urothelium. Immunohistochemical analyses on frozen and deparaffinized, formalin-fixed tissue sections have demonstrated that the Lewis X antigen is not detected in normal adult urothelium except for occasional umbrella cells. However, papillomas and transitional cell carcinomas express this blood group-related antigen in more than 90% of the cases regardless of the grade or stage of the tumor, or the blood type or secretor status of the individual. To determine the presence of Lewis X antigen on exfoliated bladder epithelial cells we used an anti-Lewis X monoclonal antibody (P-12) and the avidin-biotin-peroxidase technique on 129 bladder barbotage specimens. Of 40 controls 34 were negative for Lewis X antigen, for a specificity of 85%. The 89 bladder tumor patients consisted of 14 with papilloma, 13 with flat carcinoma in situ, 49 with transitional cell carcinoma, and 13 with a positive cytology and negative biopsy results. Of these 89 patients 76 were considered positive for Lewis X antigen, for an over-all sensitivity 85.4%. The sensitivity for cytology alone was 61.2%. However, the combination of a positive cytology and/or positive Lewis X antigen result yielded a sensitivity of 93.2%. The data suggest that immunocytological detection of the Lewis X antigen on exfoliated bladder cells enhances the detection of urothelial tumor cells, particularly from low grade and low stage neoplasms.