In a sample of healthy subjects (23-59 years old), comprising 49 non-smokers and 23 smokers, we studied the effects of smoking on DNA repair synthesis (UDS) in peripheral lymphocytes. DNA repair after UV-irradiation was measured by [3H]TdR incorporation into DNA (semi-conservative synthesis was inhibited by hydroxyurea). Compared to lymphocytes from non-smokers, lymphocytes from smokers showed significantly higher levels of UDS and higher incorporation of [3H]TdR in non-irradiated cells. We also determined the rate of DNA replication after 60 h in the presence of phytohaemoagglutinin (PHA) by measuring the incorporation of [3H]TdR during the last 20 h of culture. By irradiating the lymphocytes with UV light before setting up the cultures, we measured the inhibition of DNA replication due to damage induced by UV light. This inhibition, determined by the ratio of DNA replication in irradiated over non-irradiated lymphocytes, was less marked in smokers than in non-smokers. In lymphocytes from a portion of the donors (18 non-smokers and 19 smokers) we also determined the rate of the [3H]TdR incorporation in the absence of PHA and compared the distribution of the radioactivity between the two groups. In unstimulated lymphocytes, more radioactivity was incorporated by smokers compared to non-smokers in both conditions (UV-irradiated or mock-irradiated), while in stimulated lymphocytes the distribution of radioactivity was significantly different only after UV-irradiation.