The serum of patients with pigeon breeder's lung (PBL) contains IgG-class antibodies against pigeon serum proteins. These antibodies cross-react strongly with the sera of other avian species. Cross-reactivity, as demonstrated by precipitation, complement fixation, haemagglutination and microELISA, is also exhibited with the water-soluble proteins in hen's egg yolk and egg white. The phenomenon appears to be due to the presence of hen serum proteins in the eggs, which occur particularly concentrated among the water-soluble proteins in the yolk (‘livetins’). It is proposed that anti-egg antibodies may explain the frequent serological finding of‘false-positive’asymptomatic pigeon breeders.