The antigen structures in mycelial extracts of 4 strains of Thermoactinomyces vulgaris and 1 strain of Thermoactinomyces candidus were compared using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and silver staining. For immunological characterization, antigens were transferred electrophoretically to nitrocellulose membranes and blotted with T. vulgaris strain-specific antisera and with sera of farmer's lung patients. Cross-reactions between the strains were also studied using immunodiffusion and crossed immunoelectrophoresis. Protein components between 50 and 60 kilodaltons were found to be the most immunogenic. Patient sera showed heterogeneous responses, but all reacted with an antigen at 55 kilodaltons that was also common to all strains studied.