Sixty-nine farmer's lung patients and 28 normal controls from four countries (Finland, Switzerland, Canada and the United States) were investigated for antibody levels against 13 antigens commonly used for the screening panel for hypersensitivity pneumonitis. Of these antigens, eight were from the Medical College of Wisconsin (United States) and five were from the University of Kuopio (Finland). IgG antibodies against these antigens were studied in 97 sera using a sensitive biotin-avidin-linked enzyme immunoassay. The results indicate that the mean antibody titer against Micropolyspora faeni was highest in the United States (U.S.) followed by Finland. Both Finnish and U.S. antigens reacted almost identically against various groups of patients, although the degree of reactivity varied considerably. Higher antibody levels against Thermoactinomyces vulgaris were detected in Finnish patients than patients from other countries while patients from all four countries showed elevated levels of antibodies against T. candidus. This study demonstrates that antigens from identical species, irrespective of geographic origin, reacted similarly. However, variability between antigens of the same species was still considerably significant. Since the microbiological flora of moldy hay varies widely in different regions, the microbial species associated with the disease at a given geographical area has to be determined before selecting antigens for serological studies. The antigens currently used in various laboratories are crude preparations and need to be purified and standardized for dependable results. Until such antigens are available, all antigenic preparations used in the immunological evaluation of patients should be immunochemically characterized for their reproducibility and reliability although the ultimate goal should be to obtain standardized pure antigens for dependable immunodiagnosis of farmer's lung.