The interaction between tobacco smoking and occupational exposure to chemical agents in the causation of lung cancer is considered. The literature on the interaction of smoking with the effects of exposure to radon (10043922) daughters, arsenic (7440382), asbestos (1332214), and chloromethylethers, known occupational lung carcinogens, was reviewed to determine if smoking modifies the effect of exposure. Studies of the effects on lung cancer mortality among workers exposed to asbestos, radon daughters, and arsenic gave inconclusive or contradictory results. In some studies, smoking had no apparent effect. In studies where smoking modified the effect of exposure, the relative risk of lung cancer was higher in nonsmokers. In the case of asbestos, it is noted that the different forms of asbestos could interact differently with cigarette smoke. A study of lung cancer mortality in 51 males having moderate to heavy exposure to bis(chloromethyl)ether (542881) and chloromethyl-methyl- ether (107302) clearly showed an average relative risk of 270.3 in nonsmokers and 3.5 in smokers. The study was, however, based on an extremely small number of persons/years exposure. The authors note that the fact that nonsmokers in some studies have a higher relative risk than smokers should not be viewed as implying that smoking protects the workers from the risks of occupational exposure. Absolute, rather than relative, lung cancer risk remains generally higher in smokers than in nonsmokers. Inconclusive or contradictory findings may be due to the small sample size or lack of comparability of occupational or tobacco exposure.