With slide culture preparations of Micropolyspora faeni, Thermoactinomyces candidus and T. vulgaris used as antigens, 59 sera from patients with hypersensitivity pneumonitis and 23 sera from normal controls were studied for the presence of specific antibodies by indirect immunofluorescence and immunodiffusion methods. Of the 82 sera, 31 showed precipitins to M. faeni, 5 to T. candidus and 4 to T. vulgaris. A group of 19 sera positive to Aspergillus antigens showed questionable reactions to thermophilic actinomycetes. The 23 sera from normal individuals were negative for specific antibodies when tested by immunodiffusion. In indirect immunofluorescence studies, Ig[immunoglobulin]G antibodies in high titers were present only in the precipitin-positive sera; other antibody classes were negative or failed to show consistent differences in titers of the positive, questionable and control groups. The IgG antibody titer varied from 0-1:20 in the normal control group, 0-1:40 in the group showing questionable precipitin reactions and 1:40-1:640 in the precipitin-positive group.