The frequencies of chromosome aberrations in blood lymphocytes of a group of moderate cigarette smokers are significantly greater than in a matched group of non-smoking controls and the incidence of unambiguous exchange aberrations in the smokers was some 4 times that of controls. The incidence of sister-chromatid exchanges (SCE) in blood lymphocytes of heavy cigarette smokers is significantly greater, by a factor of around 50%, than in non-smokers. Lymphocytes from both smokers and non-smokers exposed to cigarette smoke condensate (CSC) in vitro, showed dose-dependent increases in SCE frequency with the yields from smokers being greater than those from non-smokers. The apparent enhanced effects of CSC in producing SCE in smokers' cells is considered to simply reflect the increased basal SCE levels observed in smokers' cells, although some evidence was obtained for an increased level of unscheduled DNA synthesis in cells of smokers relative to non-smokers exposed to CSC in vitro. When lymphocytes from smokers and non-smokers are exposed to mitomycin C, or to ethyl methane sulphonate, in vitro, the yields of SCE at all dose levels are significantly greater in cells from smokers. This difference between smokers and non-smokers is, however, shown to simply reflect the initial basal differences in SCE frequencies between the groups, so that cigarette smokers' cells are not intrinsically more sensitive to exposure to mutagens.