Antigens identified by indirect immunofluorescence staining and specific for sera from patients with pigeon breeders' lung or healthy pigeon breeders, have been isolated from pigeon intestinal mucus. Two antigenic peaks, one pigeon intestinal mucin and the other IgA, were separated by equilibrium centrifugation of water-soluble mucus in a caesium chloride density gradient. Antigenic positive material was identified by a modified double-sandwich ELISA, by inhibition of haemagglutination of turkey erythrocytes and by gel diffusion. Antigenic-positive material co-fractionated on gel-filtration with purified intact and papain digested pigeon mucin, both free of IgA. These studies demonstrate antibodies to two quite different antigens are associated specifically with sera from pigeon breeders, a novel antigen pigeon intestinal mucin and the previously documented pigeon IgA.