The aim of the present study was to determine which microbes and which immunoglobulin (Ig) classes should be included in tests to discriminate between patients with farmer's lung and reference persons. The sera of a group of farmer's lung patients and their spouses were measured for IgG, IgA, IgM, and IgE antibodies against a panel of farmer's lung microbes. The concentrations of IgG, IgA, and IgE antibodies were higher in patients compared with their spouses. The patients were generally positive for antibodies of several Ig classes whereas the spouses had only either IgG or IgA antibodies. A test comprising the determinations of IgG antibodies against T vulgaris and IgA antibodies against A fumigatus would correctly group 94% of the cases in the Finnish farming population. The selection of microbes for other environments needs to be determined locally.