The agglutinating activities of the peptic dimer fragment and reduced and alkylated subunits of rabbit IgM antibodies were demonstrated. By digestion with pepsin of purified rabbit IgM antibody, two kinds of fragments analogous to F(ab′)2(IgM) and Fab pep(IgM) of human IgM were obtained. The dimeric fragment, F(ab′)2(IgM), of the purified anti-azonaphthalenesulfonate (anti-azo-NS) IgM antibody agglutinated azo-NS-conjugated red cells. The agglutinating efficiency of F(ab′)2(IgM) was much lower

more »

... han that of native IgM antibody, but it was of the same order as that of IgG antibody or its peptic dimer fragment, F(ab′)2. It was carefully excluded that the activity was due to the presence of a small amount of native IgM or aggregated fragments. The reduced and alkylated subunit of rabbit anti-Salmonella IgM antibody also had agglutinating activity and its agglutinating efficiency was of the same order as that of IgG antibody. These results suggested that the dimer fragment or subunits of IgM antibody could combine with two red cells simultaneously.