In a controlled study, ten male volunteers were subjected to different smoking and passive smoking conditions. After 60 h of strictly controlled nonsmoking, five smokers were exposed to mainstream smoke only, while five nonsmokers were exposed to the gas phase of environmental tobacco smoke (ETS). In a second experiment smokers were mainstream and ETS exposed, while nonsmokers were exposed to complete ETS. Blood was drawn before and after smoking and DNA adducts were analysed from blood monocytes by the 32P-postlabelling assay, using the nuclease P1 enhancement method. We detected DNA adducts in monocytes of all probands. These adducts unrelated to smoking showed interindividual differences but only minor intraindividual changes in four samples of the same donor. After smoking interindividually variable additional adducts were visible in active smokers only. These smoking-related adducts had disappeared after 40 h of nonsmoking and reappeared again in three out of five smokers after the second smoking period. We conclude that smoking causes an interindividually variable pattern of DNA adducts in active smokers. These adducts disappear in less than 2 d, owing to the fast turnover of monocytes in the intravascular system. The effects described could not be observed in heavily exposed passive smokers.