IgM molecules in pure IgM preparations and in the corresponding sera from three patients with Waldenström's macroglobulinemia have been studied by analytical ultracentrifugation, rate zonal ultracentrifugation, agarose-polyacrylamide gel electrophoresis, and electron microscopy. One of the IgM preparations was without J chain, while the two others contained J chain. The experiments revealed that IgM with J chain consisted of pentameric molecules; whereas in IgM preparations without J chain the dominating molecules were hexamers, but also pentamers, tetramers, and even smaller molecules were found. In both types of IgM the molecules formed a wide range of aggregates, which were usually more abundant in the purified IgM than in serum. Some of the aggregates were kept together by noncovalent forces only.