A portable carbon monoxide detector (catalytic electrode) was used to record carbon monoxide concentrations in enclosed public facilities, including night clubs, bars, restaurants, stores, hospitals, and public transit vehicles. The highest concentrations of carbon monoxide were found in night clubs, where evening indoor readings averaged 13.4 ppm, which was 4.1 +/- 1.5 ppm higher than outdoor readings. In one poorly ventilated establishment, values ranged from 20 to 40 ppm, with substantial gradients around the room. Adequate ventilation and screening of restaurant employees for ischemic heart disease should prevent problems from cigarette-induced build-up of carbon monoxide. Unfortunately, much lower concentrations of other constituents of smoke cause symptoms and changes of pulmonary function in the nonsmoker. If carbon monoxide is used as an indicator of cigarette smoke accumulation, concentrations should not exceed ambient readings by more than 5 ppm.