Descriptions have previously been given of the physiochemical and immunobiological behaviour of IgG co-precipitating antibodies, isolated from different animal species, compared with that of the precipitating ones belonging to the same immunoglobulin class. From those investigations it seems reasonable to assume that only one of the combining sites present in the two binding arms of the co-precipitating antibody molecule firmly binds the antigen. Consequently, the antibody does not form insoluble Ag-Ab complexes. The peptide maps, diagonal peptide maps and high voltage electrophoresis developed with peptides obtained after previous reduction and radioactive alkylation did not show differences between sheep IgG1 precipitating and co-precipitating antibodies. When diffused against rat anti-sheep IgG1 precipitating antibody serum, sheep IgG1 precipitating antibody gave a band of precipitation which was identical to the band given by sheep IgG1 co-precipitating antibody. Similar results were obtained when rat anti-sheep IgG1 co-precipitating antibody serum was used for precipitation. By immunodiffusion with cross absorbed sera no antigenic differences between the two antibodies could be demonstrated. The results obtained indicate that the different behaviour of precipitating and co-precipitating antibodies when interacting with antigen would probably not be a consequence of differences in the primary structures of their constant fragments. Both antibodies would belong to the same class, sub-class or type of immunoglobulin.