The immunoglobulin J (joining) chain plays an important role in the assembly of polymeric immunoglobulins (dimeric IgA and pentameric IgM) and in the selective transport of these across epithelial cell layers. The primary structure of the bovine J chain has been determined by sequencing of three cDNAs. The cDNA has an open reading frame of 471 nucleotides encoding a putative protein of 157 amino acids. The 3' untranslated region consists of 698 nucleotides and a poly(A) tail. The 5' untranslated region and the promoter were isolated from a genomic clone. By comparison with the murine J chain gene, the 5' untranslated region was predicted to be 37 bp, giving the bovine J chain cDNA a total length of 1,206 bp. This size was confirmed by Northern blot analysis of total RNA from colon and mammary gland. The amino acid sequence of the bovine J chain shows extensive homology with the J chain from human, mouse, rabbit, and bullfrog. Analysis of the J chain secondary structure showed a high propensity for forming beta-sheets. An alignment of the predicted secondary structure of the J chain from bovine, human, mouse, rabbit, and bullfrog revealed a highly conserved "beta-profile." The promoter sequence of the bovine J chain gene is presented and shown to contain a conserved interleukin-2 (IL-2)-responsive element previously characterized in the murine J chain gene.