Carcinoembryonic antigen-like substances were measured in urine samples from 236 apparently healthy, asymptomatic individuals. Twenty-four-hour collections were submitted by 177 subjects, and random specimens were collected from 59 others. In addition to CEA measurements, bacterial colony counts and cytologic examinations were performed on the latter. Baseline values were established for males and females and data were analyzed for the effects of age, smoking, time of collection, and bacterial colonization. Results indicated that levels of CEA-like substances in the urine could be greatly altered by bacterial conlonization. Lesser effects were noted with age and time of collection. Smoking had no demonstrable effect. These variables should be considered for meaningful interpretation of urinary CEA values of patients suspected of urothelial malignancy.