A study was made of the prevalence of serum precipitins to Micropolyspora faeni, Thermoactinomyces vulgaris, Aspergillus fumigatus, and pigeon serum in population groups suspected to be at high risk for the development of hypersensitivity pneumonitis. Pigeon breeders' sera contained precipitins mainly to pigeon serum (38%) and A. fumigatus (18%). Occupants of homes in which forced air heating systems were investigated for the presence of microorganisms reacted mostly with M. faeni (13%) and A. fumigatus (8%). Individuals from environments where several cases of hypersensitivity pneumonitis were discovered reacted largely with M. faeni (28%) and T. vulgaris (21%). Sera supplied by physicians from patients with respiratory symptoms reacted primarily to A. fumigatus (24%) and to a lesser extent to M. faeni (16%) and T. vulgaris (9%). The results indicate that the prevalence of precipitins to the tested antigens is not uniform and may be influenced by the environment. Furthermore, the prevalence of precipitins in groups at risk is greater than previously reported for the normal population.