J chain is a polypeptide of molecular weight (Mr) approximately 15,000 common to human dimeric IgA and pentameric IgM. These immunoglobulin polymers show a high affinity for secretory component (SC) in vitro, a feature that, in some studies, has been claimed to be a function of the J chain. SC is a glycoprotein of Mr approximately 80,000 which is expressed on the basolateral surfaces of secretory epithelial cells where, according to a current hypothesis, it may act as a receptor for dimeric IgA and pentameric IgM which are selectively transported through secretory epithelial cells into exocrine fluids. Previous studies, however, have not excluded the possibility that secretory cells express isotype-specific Fc receptors for IgA and IgM which may be involved in epithelial transport. We now report that the adsorption of immunoglobulin polymers to SC-expressing epithelial cells depends solely on a J chain-determined binding site. This finding lends biological significance to the striking J-chain expression shown by immunoglobulin-producing immunocytes in secretory tissues.