We have characterized the early biosynthetic forms of secretory component (SC). SC is synthesized in various glandular epithelial cells and functions in transepithelial transport of certain polymeric immunoglobulins. In rabbit, mature SC is known to be heterogeneous, consisting of two or three immunologically related glycoproteins. Using translation of mRNA from rabbit mammary gland or liver in a wheat germ cell-free system supplemented with dog pancreas microsomal vesicles, we discovered that the translation products of SC mRNA include at least four distinct polypeptides. Moreover, we found that all four polypeptides are synthesized not as soluble secretory forms but as considerably larger transmembrane forms that are core-glycosylated and asymmetrically integrated into the dog pancreas microsomal vesicles in a translation-coupled manner. We therefore conclude that the mature secreted forms of SC are endoproteolytic fragments derived from and comprising the ectoplasmic portion of the transmembrane precursor forms. The detection of transmembrane precursor forms for mature SC provides a protein structural basis for their function as receptors mediating transepithelial transport of immuno-globulin molecules.