Immunochemical studies are described in an unusual case of Waldenström's macroglobulinemia. Two monoclonal Igs (whole IgG₁/ and IgG₁/ half molecules) occurred in the serum in addition to the IgM monoclonal protein. Protein electrophoresis of the serum showed a monoclonal component in the region, and the immunoelectrophoresis allowed detection of a monoclonal IgM/ and another abnormality represented by a double precipitin line in serum and urine, observed when antiserum anti IgG was used. The abnormal proteins were purified and further analyzed. The IgG-related proteins were whole four chains IgG monoclonal molecules, 1/2 IgG monoclonal molecules, composed of one heavy and one light chain, and residual polyclonal IgG. The half molecules were antigenically deficient with respect to normal IgG. The idiotypic analysis showed that the three monoclonal proteins shared idiotypic determinants. This patient had clinical and morphological findings of Waldenström's macroglobulinemia and, as observed in other cases, the formation of half molecules was not associated with a distinct clinical syndrome.