The requirements for IgM assembly and secretion were evaluated by introducing a constitutively expressed J-chain cDNA into lymphoid and nonlymphoid cell lines expressing the secretory form of monomer IgM. Assays of cell lysates and supernatants showed that only secretory monomer IgM is required for the synthesis and secretion of hexamer IgM, whereas J chain, as well as the secreted form of monomer, is required for the synthesis and secretion of pentamer IgM. Moreover, J chain facilitates the polymerization process so that pentamer IgM is preferentially synthesized. Other components of the polymerization process were found to be shared by all the cell lines examined, whether the cells were of lymphoid or nonlymphoid origin and had a rudimentary or developed secretory apparatus. These results identify monomer IgM and J chain as the two components that determine the B-cell-specific expression of IgM antibodies and, thus, as the appropriate targets for therapeutic regulation of IgM responses.