IgM from trypanosome-infected rabbits was digested with trypsin under different conditions to obtain Fab mu or Fc5 mu fragments suitable for analysis with anti-allotype and anti-isotype antibodies. The Fab mu but not the Fc5 mu fragment was shown to have the n-locus allotypic specificities, n80, n81, n82, n83 and n87, characteristic of the IgM class of immunoglobulins. Thus, the n82 and n83 allotypic specificities, conformationally dependent on the a VH locus for expression, and the n80, n81 and n87 allotypic specificities, independent of the a VH locus for expression, are in either the CH1 or CH2 domain of IgM heavy chains. In addition, two high-affinity mouse monoclonal antibodies (MoAbs) specific for IgM and able to bind IgM in direct-binding radioimmunoassays were produced and characterized. One MoAb (3C1) was specific for an isotypic determinant (epitope) in the Fab mu fragment, presumably in the CH1 or CH2 domain, whereas another MoAb (8C2) was specific for an isotypic epitope in the Fc5 mu fragment, presumably in the CH3 or CH4 domain. The proximity of the n-locus allotypic specificities (CH1 or CH2 domain) to the VH domain is consistent with the finding that some IgM allotypic specificities are expressed only in conjunction with certain a VH locus allotypic specificities.