The concentrations of IgA, IgM and IgY were measured in gallbladder (GB) bile and serum from chickens of various ages. The ontogeny data suggested that IgA and IgY were synthesized by the GB. Furthermore, the chicken GB becomes lymphoid 3–8 days post-hatch and contains distinct foci of lymphocytes by 7 weeks of age. That the GB can synthesize and secrete IgA was shown in two ways. IgA-containing cells were demonstrated by immunofluorescence and in vitro studies with adult GB explants showed the synthesis and secretion of IgA and also IgY and IgM. Immunoglobulin class suppression experiments and quantitation of IgA in sera from selected sites supported the concept of synthesis and secretion of IgA by the GB. Several lines of evidence also support the concept of some bile IgA being derived from serum. Introduction of antigen directly into the GB was shown to stimulate antibody formation, and this antibody was detectable in bile and serum. A very unusual finding, in one animal, was the presence of a normal secretory IgA concentration in the presence of undetectable serum IgA. These data indicate that the GB is an integral part of the secretory immunological system and suggest that aberrations in GB-immune function may result in pathological clinical manifestations.