Two IgG4 paraproteins having heavy chains of normal molecular weight are shown to be antigenically distinct in their reactivity profiles with 18 monoclonal antibodies having specificity for the IgG2 or IgG4 subclasses. One protein expresses IgG2 and IgG4 epitopes within the C gamma 2 domain. A second protein is deficient in the expression of IgG4 Fc-specific epitopes. These proteins will be of value in defining the structural basis of Fc effector functions and individual epitope expression.