Serum levels of 13 individual complement component and control proteins were measured in 25 male cigarette smokers and 25 male non-smokers. A significant elevation of the mean serum levels of C5, C9 and C1-inhibitor was demonstrated for the smokers while levels of C1q, C2, C4, C3, C6, C8, Factor B, properdin, C3b inactivator and beta 1H did not differ significantly between the two groups. Serum level of C9 in individual smokers correlated significantly with both the present amount of consumption and cumulative consumption, while C5 level correlated with present consumption. The complement profile in the cigarette smoker is similar to that observed in certain inflammatory conditions. The elevation of C5, C9 and C1-inhibitor levels may reflect the presence of a low grade inflammatory process in the cigarette smoker.