The stoichiometry of J chain in pentamer IgM has been determined by measuring the radiolabeled thiols in the constituent chains after complete reduction and alkylation of the polymer. One mole of J was found to be disulfide bonded to 1 mol of pentamer. The linkage of J chain in IgM has been determined by correlating the J disulfides cleaved with the subunits released after limited reduction and alkylation of the polymer. The analyses showed that: ( a ) Significant amounts of monomers, as well as small quantities of dimers, trimers, and tetramers, were generated by the reducing conditions employed. ( b ) The number of J disulfide bonds broken did not correspond to the extent of depolymerization. ( c ) No J disulfides were cleaved in the J-containing dimer products of the limited reduction. These data demonstrated that the J chain is located as a disulfide clasp between two of the IgM monomer subunits. From the observed linkage, the assembly of IgM is postulated to proceed by a series of sequential disulfide exchanges beginning with the formation of the J-containing dimer.