Fragmentation of IgM antibodies may be necessary because of the large molecular weight of the native molecule (900 kDa). IgMs fragments resemble IgG in size and structure, but they may have a decreased binding affinity. The Fc portion of IgM can have powerful biological effector functions such as complement activation. Because some T cells have receptors for IgM, it may be desirable to produce fragments of IgM for both cytotoxicity studies and for in vivo use. A protocol is presented for digestion of IgM with pepsin to produce F(ab')(2)micro. The fragment can be reduced to produce the monovalent F(ab')u, if desired. IgM can also be reduced and alkylated in a single step, as described, using cysteine to produce IgMs, the bivalent monomer or subunit of IgM.