Simultaneous numerical and functional studies of circulatory lymphocytes were undertaken in healthy non-smoking and cigarette-smoking volunteers. The smokers all had light to moderate histories of less than 50 pack years. By contrast with non-smokers (n = 32), the smokers (n = 14) had a significant increase in the total number of lymphocytes, surface immunoglobulin bearing (sIg+) cells, total T-cells (T3+) and T helper-inducer cells (T4+), and a trend of increase in T suppressor-cytotoxic cells (8+). These changes differ from those in heavy smokers who have been reported to show significantly increased T suppressor-cytotoxic but significantly decreased T helper-inducer cells. Although the proportions of T-cell subsets did not differ significantly in the light to moderate smokers compared with non-smokers, in vitro T-suppressor function against the Ig-secreting response of allogeneic B-cells to pokeweed mitogen (PWM) stimulation was significantly impaired. The proliferative response of T-cells to phytohaemagglutinin (PHA) was, however, similar in both groups. This suggests that smoking may exert a selective influence upon a subset of T suppressor cells. In cytotoxicity assays, smokers showed a significant decrease in natural killer cell (NK) activity but not in antibody dependent cellular cytotoxicity (ADCC). It appears that these alterations are reversible since a group of ex-smokers (n = 10) were indistinguishable from our non-smoking group in all studies. The implications regarding the link between smoking and increased susceptibility to infection and malignancy are discussed; and these findings should be borne in mind in basic studies of lymphocytes.