Serum IgE levels in healthy blood donors who had no history of atopy were measured by a paper-disc RIA and analyzed according to the donors' smoking habits. The IgE geometric mean for regular smokers was 41.7 IU/ml, which was significantly higher than that for nonsmokers (19.3 IU/ml) or rare smokers (22.7 IU/ml). Whereas 28% of smokers had IgE levels greater than 200 IU/ml, none of the rare smokers or nonsmokers did. IgE levels in smokers showed a moderate inverse correlation with the degree of smoking. The mean IgE level was 189.8 IU/ml in those who smoked 1-9 cigarettes/day but only 32.8 IU/ml in those who smoked 10-19 cigarettes per day and 11.1 IU/ml in those who smoked 20 or more cigarettes/day. The number of years a person smoked did not seem to significantly influence the IgE level. The mean IgE level in ex-smokers (50.5 IU/ml) was much lower than in current light smokers but was still higher than in nonsmokers. There was a moderate inverse correlation between IgE levels and duration of cessation of smoking. Our data suggest a characteristic pattern for the influence of cigarette smoking on serum IgE level, namely, a striking rise associated with light smoking and a remarkable drop in heavy smokers, and such changes seemed reversible after the habit was stopped. Smoking status, therefore, appears to be an important consideration in interpreting serum IgE levels and in revising the "norms" of IgE levels.