Cigarette smoke is a complex chemical mixture which displays a number of biological activities including mutagenesis. The absorption of smoke components by the lungs results in their systemic distribution, which then leads to the presence of mutagens in the urine. The detection of mutagenic substances in the urine by short-term bioassays has been suggested for use in monitoring human exposure to various carcinogenic/mutagenic substances. In this connection, we felt that further studies on the mutagenic activity present in cigarette smokers' urine were needed. In the present studies, the dose-response relationship between the number of cigarettes smoked versus urine mutagenic activity and the persistence of mutagens in the urine after the cessation of smoking have been examined. The studies were carried out on a group of smokers and non-smokers in Lexington, Kentucky. Although a dose-response relationship (urine mutagenicity versus the number of cigarettes smoked per day) was not observed in the group of smokers assayed, a linear dose-response relationship was detected in an individual smoker who smoked a varying number of cigarettes over eight successive days. In another smoker who ceased smoking, mutagens were detected in the urine for one week. The urine mutagenicity data from a group of smokers and non-smokers should prove useful in extending this technique to the monitoring of other environmental exposure.