Transcytosis of polymeric Ig (pIg) by mucosal epithelial cells is mediated by the polymeric Ig receptor (pIgR). Here we describe the characterization of a 3095-bp mouse pIgR cDNA, which encodes a protein of 771 amino acids. Northern blot analysis detected a single mouse pIgR transcript of 3.9 kb, expressed at high levels in small intestine and liver, and at low levels in lung. Alignment of the amino acid sequences of mouse, rat, human, bovine, and rabbit pIgR revealed that functional regions of the molecule are conserved across species. In the extracellular region, conserved motifs include: a 23-amino acid pIg-binding site, 11 intradomain disulfide bonds, consensus sites for N-glycosylation, and a putative cleavage site at which the extracellular region of pIgR (secretory component) is released from the plasma membrane. A 10-amino acid sequence within the transmembrane region is highly conserved, possibly reflecting a mechanism for transmitting signals from the extracellular region to the cytoplasmic tail. Conservation within the cytoplasmic tail of pIgR is clustered in motifs that mediate polarized sorting, endocytosis, and transcytosis.