The cysteine at position 575 of the immunoglobulin mu heavy chain is thought to provide the only disulfide bonds joining the monomer subunits of mouse polymeric IgM. The importance of this cysteine in the assembly of polymeric IgM was investigated by using site-directed mutagenesis to produce mu chains with serine at position 575. Thirty percent of the secreted mutant IgM was covalently assembled polymer implying that cysteines other than Cys575 can form inter-subunit disulfide bonds. The polymeric IgM lacked J chain, mediated complement-dependent cytolysis and appeared to have a higher molecular weight than conventional IgM pentamers, as judged by sucrose gradient sedimentation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis mobility. Electron microscopy revealed that the mutant IgM molecule contained six subunits. Wild-type IgM, while synthesized predominantly as a pentameric molecule, was assembled in at least two other forms, which were distinguished by their electrophoretic mobility. The apparently higher molecular weight forms of wild-type IgM include hexameric molecules which, like the hexameric mutant IgM, contained much less J chain that the pentameric form and were 20-fold more efficient at activating complement-dependent cytolysis.