Whereas fifteen pigeon fanciers suffering from extrinsic allergic alevolitis from avian dust had high titres of antibodies against pigeon antigens, antibodies were not demonstrable, even by immunofluorescence, in the serum of a symptomatic individual exposed to minimal amounts of avian dust. Following exposure to larger quantities of pigeon dust inhalation challenges, a low titre of antibodies appeared, but disappeared again after avoidance of the allergen. Cell-mediated immunity was elevated in the lymphocyte transformation test and also decreased after avoidance of allergen contact. Therefore, it seems likely that the antibody is not the only mediator of pigeon breeder's lung. Inhalation challenges and T cell-dependent immune reactivity may reveal more avian dust sensitive individuals suffering from fibrosis without the typical history of extrinsic allergic alevolitis and without detectable antibodies.