Antibodies to antigens in humidifier water were detected by double immunodiffusion in 30 of 63 (47.6%) persons who were exposed to aerosols from a water humidification unit in a cigar plant, whereas no antibodies could be detected in 49 unexposed blood donors (P less than 0.001). The presence of antibodies could not be related to fever or pulmonary symptoms (cough, expectoration, dyspnoe). Antibodies were found in 14 (93.3%) of 15 nonsmokers and in only 13 (31.7%) of 41 smokers (P less than 0.001), and the titres were highest in nonsmokers. Serum IgG and IgA levels were higher in nonsmokers than in smokers, and the variances within the groups were significantly different (F less than 0.05 and F less than 0.05, respectively). The mean serum IgM values were not significantly different in the two groups. Antibodies to Candida albicans and Escherichia coli 04 and 075 were detected with equal prevalences and titres in smokers and nonsmokers. These findings suggest that tobacco smoking may suppress the humoral immune response to inhaled antigens but not to antigens which are supposed to be absorbed through membranes other than those of the bronchopulmonary system. They may partly explain the reported increased incidence of allergic alveolitis in nonsmokers.