A sutstance immunologically similar to carcinoembryonic antigen (CEA) was demonstrated in urothelial bladder carcinoma cells. When indirect immunofluorescence with specific anti-CEA antisera was used, CEA-containing cells were seen in 18 out of 40 cases in 5% to 30% of the cells. Among patients with tumor cells of well differentiated morphology, 61% had CEA-containing cells, compared with only 24% of patients with a poorly differentiated tumor. Microfluorometry of single cells was performed in six cases to estimate the range of CEA content. The mean fluorescence intensity with anti-CEA antiserum was three to six times that of the same tumor cell population stained with non-immune rabbit sera. This fluorescence was decreased when the anti-CEA anti-sera were incubated with CEA but not with nonspecific cross-reactive antigen. The results show a wide range of CEA antigen content in exofoliated bladder tumor cells. In addition to proliferative status and differentiation, quantitative CEA measurements give further possibilities to study characteristics of tumor cell populations.