Antibodies to whole pigeon serum (PS) and pigeon dropping extract (PDE) were investigated in pigeon breeders and controls by an enzyme-linked immunosorbent assay (ELISA). The optimal antigen concentration was in the range of 3-30 microgram protein/ml for both PS and PDE. PDE antibody titres greater than or equal to 10240 were found in three (75%) of four patients with pigeon breeders' disease (PBD) and in 19 (13.6%) of 140 other pigeon breeders (P = 0.011). PS antibody titres greater than or equal to 10240 occurred in all four (100%) PBD patients and in only seven (5.0%) of 140 other breeders (P = 0.0002). In 85 blood donors antibodies in low titres against PS and PDE occurred in 4.7% and 7.1%, respectively. By immunodiffusion three or more precipitin lines were found more often in PBD patients than in other breeders (P = 0.0006), and the highest ELISA antibody titres occurred in patients with precipitating antibodies. No correlation between P blood group phenotypes or anti-P1 antibodies and respiratory symptoms or antibodies to pigeon antigens could be demonstrated.