This study employed quantitative immunoelectrophoretic techniques, on sera from confirmed cases of pigeon breeders' disease (PBD), to investigate the antigenicity of a pigeon bloom extract, implicated as a sensitizing agent in this disease. On crossed immunoelectrophoresis the maximum number of antigenic components identified was 29 for the bloom compared to 10 for pigeon serum. A major component was shown to be closely related to pigeon IgA, and demonstrated partial crossreactivity to the pigeon IgG. This component also showed identity with the major component of a pigeon droppings extract, considered to be derived from intestinal IgA. Only trace amounts of serum albumin were detected and most other bloom components were not serum-related. Although greater overall antigenic similarity was found to pigeon droppings extract, at least three of the bloom components appeared to be specific. The bloom extract also contained a low amount of an alpha-techoic acid-like component, causing some non-specific reactivity. Pigeon feather dust or 'bloom', like pigeon droppings, is therefore a potent source of antigens associated with PDB--pigeon IgA being a major component of both antigens.