The incidence and characteristics of P1-antibodies in Scottish pigeon fanciers has been assessed. Of 223 individuals participating in the study, 23.3% were P1-negative and of these, 53.9% had detectable P1-antibodies. These antibodies were shown to belong to the IgM class and the highest serum levels were found in those pigeon fanciers with symptoms of respiratory disease who were sensitized to pigeon antigens. Of pigeon fanciers'; sera, 10.3% gave false positive reactions for the Hepatitis B surface antigen in a reversed passive haemagglutination test. The reactions were most prevalent in sensitized fanciers and were shown to be directed not against the absorbed HBsAg antibody, but against the turkey red cells used as carriers in the test. No relationship was observed between the existence of P1-antibodies and false-positive hepatitis reactions. It was concluded that the inhalation of avian antigens by pigeon fanciers can induce the production of at least three distinct populations of antibodies. No evidence was obtained to imply any special role for P1-antibodies in the aetiology of pigeon fanciers' disease.