The receptor that mediates the specific uptake and intracellular transport of dimeric immunoglobulin A (IgA dimer) in mucosal and glandular epithelia is identical with a transmembrane precursor of secreted secretory component. During transport, the IgA dimer receptor (membrane SC) is cleaved into two domains, a membrane anchorage peptide and secreted secretory component. We have produced monoclonal antibodies with distinct specificity against both domains of the rabbit IgA dimer receptor. Two mouse hybridoma lines were obtained by fusion of SP2/0 myeloma cells with spleen cells from mice immunized with purified receptor from rabbit liver and by screening of culture supernatants in an immunoprecipitation assay with radiolabeled receptor. One antibody, designated anti-SC 303, reacts both with membrane and secreted SC and is therefore directed to a determinant on the secreted domain of the IgA dimer receptor. The other antibody, anti-SC 166, unable to interact with secreted SC, recognizes the membrane domain of the receptor. We discussed the unique precursor relationship between a cell-surface receptor and a secreted protein and its implications in the IgA dimer transport system.