J chain is associated with pentameric IgM and dimeric IgA via disulfide bonds involving the penultimate cysteine residue in the secretory tailpiece of the μ or the α heavy chain. We have investigated the structural basis for incorporation of J chain by analyzing several IgM mutants, IgA mutants and IgG/IgM hybrid molecules. IgM mutants with the μ secretory tailpiece replaced by the α secretory tailpiece and/or Cys414 replaced by serine incorporated J chain, although in reduced amounts correlating with reduced pentamer/polymer formation. In addition to pentamers, tetramers of IgMC414S contained J chain, while no J chain was associated with smaller polymers or hexamers of IgM. An IgA/IgM hybrid tailpiece abolished J chain incorporation to pentameric IgM. Analysis of IgG molecules that have added a secretory tailpiece and/or have IgM domain replacements showed that J chain incorporation depends on regions of the C_μ4 domain in addition to the tailpiece. Features of the C_μ3 domain other than Cys414 also play a role in efficient formation of pentamers and J chain incorporation, while the C_μ2 domain is not specifically required. By analysis of two IgA mutants that formed larger polymers than IgAwt, we found J chain equally incorporated into dimers, trimers, tetramers and pentamers. Thus, the results show that J chain incorporation into IgA does not depend on the polymeric structure, while J chain incorporation into IgM is restricted to certain polymeric conformations.