A survey of the frequency of precipitins to the antigens of the thermophilic actinomycetes and Aspergillus species was conducted on serum samples from 1,045 farmers obtained at a 3-day exposition on modern farm equipment and farming practices in central Wisconsin. Each farmer filled out a questionnaire including socioeconomic information, lung disease history, exposure history, and smoking history. Precipitins were detected by the double-diffusion method. The antigen panel included eight thermophilic actinomycetes species and a mixture of Aspergillus species. Precipitins were found in the sera of 93 famers (8.9%). The distribution of positive precipitins was: Micropolyspora faeni 63 (67.7%), Thermoactinomyces vulgaris 7 (7.5%), Thermomonospora viridis 2 (2.2%), M. faeni + T. viridis 16 (17.2%), M. faeni + T. vulgaris 1 (1.1%), Aspergillus species 4 (4.3%). Of all the parameters tested for in the questionnaire, those with positive serology differed significantly from the whole population only in that a higher proportion of the positives reported exposure to silo gas and illness after uncapping silos. Comparison of the size of the farm and the number of dairy cows in the state of Wisconsin with the population samples indicated that the sample population was skewed toward those with larger farms and larger dairy herds. This study confirms that a significant proportion of the farm population in Wisconsin does have precipitins to the microorganisms associated with farmer's lung disease. Follow-up studies to establish the relationship between the positive precipitin reactions to the presence of clinical disease are now under way.