The levels of circulating IgG antibodies to Aspergillus umbrosus, Aspergillus fumigatus, Thermoactinomyces vulgaris, and Micropolyspora faeni were determined by enzyme immunoassay in 197 subjects selected for a study of farmer's lung (FL). The material consisted of five study groups: 37 patients with clinically confirmed FL, 31 spouses of the patients, 44 immediate relatives of the patients, 35 immediate relatives of the patients' spouses, and 50 unrelated people who were spouses of the 79 people in both relative groups. The mean titres of IgG antibodies to all four microbes were highest in patients with clinically established FL. In the other groups the mean titre of Aspergillus umbrosus, a mould found much more frequently in Finnish farm environments than other moulds under study, was significantly higher (p less than 0.01) in the relatives of FL patients than in other people. This finding remained irrespective of whether the subjects had suffered from FL symptoms or not or whether they worked or lived on the same farm as the patient or on a different one. The difference in the mean titre was not due to the differences between the study groups in age, sex, smoking habits, atopic background, frequency of handling of plant materials, or time interval from the most recent handling of visibly mouldy hay. The results imply that genetic factors may be important in the IgG antibody response to microbial antigens associated with FL.