Secretory immunoglobulins A (SIgA) and M (SIgM) were investigated in 20 sera containing high levels of monoclonal polymeric IgM or IgA. In the sera of patients suffering from Waldenström's macroglobulinemia (WM), the level of SIgA was found to be low, whereas that of SIgM was extremely high. Reciprocally, in the multiple myeloma (MM) patients, SIgA were increased and SIgM were dramatically decreased. Electrophoretic analysis showed these SIgA and SIgM to have the same monoclonal pattern as the corresponding paraprotein. Hence these molecules must originate from the malignant clone. The most likely mechanism involved is an intravascular formation of the secretory-like immunoglobulins. Free secretory component (SC) could diffuse passively from the digestive lumen and bind the circulating myeloma polymeric immunoglobulins. Such a possibility of in vivo binding of free SC to IgM and IgA polymers leads to a reconsideration of the secretory origin of SIgM and SIgA in normal human serum.