Hypersensitivity pneumonitis resulting from environmental exposure to Saccharopolyspora rectivirgula (Micropolyspora faeni) among farmers has been well recognized. The diagnosis of the disease depends on demonstration of circulating antibodies against S. rectivirgula. However, dependable pure antigens are not available for serodiagnosis. In the present study we have employed hybridoma technology to obtain monoclonal antibodies against S. rectivirgula antigens. These monoclonal antibodies were employed to purify antigens through affinity chromatography. When tested in ELISA, high levels of antibodies were demonstrated against these antigens in farmer's lung patient sera compared to exposed but asymptomatic individuals from the same household. In Western blots patient sera reacted with components of crude antigens with molecular masses of 28, 35, 60, 65 and 68 kD and 4 components above 100 kD, while the monoclonal antibodies reacted only with the 60-kD protein. These purified antigens can be used as reliable reagents in the specific diagnosis of farmer's lung disease.