Twenly-one different biotinylated lectins were used to recognize the carbohydrates of Aspergillus umbrosus, one of the most common microbes that patients with farmer's lung in Finland are exposed to. The glycoprotein fraction of A. umbrosus was bound especially well by Concanavalin A and consisted mainly of carbohydrates mannose and glucose. The carbohydrate fraction of A. umbrous antigens were isolated from the crude extract of A. umbrosus with Con A-Sepharose affinity chromatography. Serum IgG antibodies to A. umhrosus mannoprotein fraction were determined in 57 patients with farmer's lung, 10 asymptomatic exposed farmers and 10 healthy controls by enzyme linked immunosorbent assay. Candida albicans and Saccharomyces cerevisiae mannan were used as controls. Patients had high levels, whereas asymptomatic exposed farmers showed moderate, and healthy controls low levels, of IgG antibodies to the A. umbrosus mannan/mannoprotein fraction. There were no significant differences in the mean antibody levels between the patients and controls against C. albicans or S. cerevisiae mannose fraction although in all groups more antibodies were detected against C. albicans than S. cerevisiae.