J chain is a polypeptide of molecular weight (M_r) ∼ 15,000 common to human dimeric IgA and pentameric IgM^1,2. These immunoglobulin polymers show a high affinity for secretory component (SC) in vitro^3,4, a feature that, in some studies, has been claimed to be a function of the J chain^5,6. SC is a glycoprotein of M_r ∼80,000 which is expressed on the basolateral surfaces of secretory epithelial cells^7,8 where, according to a current hypothesis, it may act as a receptor for dimeric IgA and pentameric IgM⁹ which are selectively transported through secretory epithelial cells into exocrine fluids^10,11. 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^12–14.