Rabbit antibodies (7S) degraded by papain into univalent 3.5S fragments fail to fix complement when they combine, but do not precipitate, with the homologous antigens. Divalent 5S fragments obtained by pepsin digestion (composed of fragment I linked to II, but lacking fragment III) also fail to fix complement although they precipitate with homologous antigens. The amount of specific precipitate formed by the 5S antibody fragment is not increased by exposure to complement