An IgM protein that reacted with human IgG was isolated from the plasma of a patient with Waldenstrom's macroglobulinemia. The IgG binding site was localized to the F(ab')2μ fragment of the IgM molecule and the activity was shown to be specific for structures present in the Fcγ fragment of the IgG. μ and kappa chain fractions were isolated from the IgM after reductive cleavage. Recombination of equimolar quantities of these fractions through both non covalent and disulfide bond formation resulted in significant levels of anti IgG activity, as measured by the agglutination of IgG coated tanned erythrocytes and precipitation of heat aggregated IgG. This activity was detected shortly after recombination and increased to a maximum after 2 or 3 wk of storage at 2°C. Ultracentrifugal studies demonstrated that the active products were heterogenous and sedimented over the range of approx. 16S to 40S. Neither chain was active when carried through the recombination procedure in the absence of the other. Maximum anti IgG activity was recovered from mixtures containing equimolar or greater quantities of the kappa chains. Also, only insignificant levels of activity were detected upon substitution of μ or kappa chains from another Waldenstrom's protein that did not bind IgG. The results demonstrated that active molecules were regenerated from the separated polypeptide chains of the IgM and indicate that both chains contributed to the formation of the active site.