The primary structure of the murine J chain was investigated by sequence analysis of the J chain cDNA inserts from two independently cloned chimeric plasmids. The sequence data showed that (i) the two cDNA inserts accounted for all but approximately 100 5' nucleotides of the J chain mRNA and (ii) the J chain mRNA encodes a prepeptide of at least 23 amino acids, a mature protein of 137 residues, and an untranslated 3' region of 707 nucleotides exclusive of the 3' poly(A) tract. The amino acid sequence deduced for the mature mouse J chain was found to be 74% identical with that previously determined for the human J chain. By analyzing the conserved features of the sequence, a two-domain structure was generated for the J chain which correlates well with its functions in the polymerization of IgM and IgA. Moreover, by comparing the homologies of the J and heavy chains in mouse and man, evidence was obtained that the structures involved in polymerization are the most conserved elements of immunoglobulin molecules.