The frequency of precipitating antibodies to Aspergillus, thermophilic actinomycetes, and pigeon serum was studied in hospitalized patients and in employees of the state of Wisconsin. Precipitins to Aspergillus were detected in 9 per cent of the hospitalized patients and 3 per cent of the state employees. The majority of the serologically reactive hospitalized patients were diagnosed as having carcinoma with metastases, lymphoma, or leukemia. No evidence of hypersensitivity pneumonitis or invasive aspergillosis was seen in these patients. The frequency of antibodies to thermophilic actinomycetes (3 per cent) and pigeon serum (1 per cent) was similar in both groups. Women from 17 to 25 years of age and men from 52 to 66 years of age demonstrated an increased incidence of positive precipitins.