The repolymerization of human IgM following mild reductive cleavage was studied as a model for intracellular polymer assembly. Repolymerization was found to require the presence of J chain and a disulfide exchanging system which could be furnished either intrinsically by the use of the monofunctional thiol mercaptoethylamine or extrinsically by the inclusion of a protein-mercaptan mixed disulfide, and/or a disulfide exchanging enzyme. The degree of repolymerization was dependent on the extent of monomer reduction and the product covalently incorporated one J chain per five monomer units. Disulfide exchanging enzyme probably served as a source of mixed disulfides rather than as an enzymatic catalyst of the reaction. These results are discussed in terms of a tentative mechanism for IgM polymerization.