Pigeons were immunized with bovine serum albumin and the subsequent antibody responses were characterized by several immunologic techniques. The antibody responses were delayed in onset, and 6 of 17 pigeons failed to produce detectable hemagglutinating antibody until after a second or third injection. Only 4 of 17 birds produced precipitating antibody. Mercaptoethanol-sensitive antibody persisted after the primary stimulus as the hemagglutination titers of some sera collected after the third injection could still be reduced by treatment. Immunoelectrophoresis and autoradiography revealed the presence of antibodies with mobilities similar to human IgG and IgM. A heat labile skin-sensitizing antibody was detected and had a minimum latent period of 24 hr and other characteristics similar to human homocytotropic antibody.