The sera of patients with pigeon breeders' disease usually contain both precipitating and complement-fixing antibodies to antigens in pigeon droppings or pigeon serum. These antibodies are not equivalent. Both types of antibody could be detected by means of an ELISA technique, using either pigeon serum or pigeon dropping antigens in the solid phase. The good correlation between the ELISA results and the complement consumption assay indicates that the ELISA technique may represent a one-step assay for assessing both precipitating and complement-consuming IgG-class antibodies, thereby identifying both symptomatic patients and breeders potentially at risk. IgM antibodies appeared to be of minor significance in the serology of pigeon breeders' disease.