We report the molecular cloning and characterization of SMGC, a major secretory product and a marker of the type I (terminal tubule) cells of the neonatal rat and mouse submandibular gland. SMGC is expressed in the submandibular gland at high levels through postnatal day 20, but in the adult is present only in some intercalated duct cells. Rat and mouse SMGC have deduced molecular weights of 67.8 and 74.4 kDa, respectively, are 37% Ser+Gly+Thr, and contain tandem repeats of between 8 and 60 amino acids. Secreted SMGC visualized by SDS-PAGE and silver staining is 89 kDa in rat and 105 kDa in mouse, although Western blot analyses with anti-SMGC antisera demonstrate multiple additional lower molecular weight forms. Contributions to the heterogeneity of SMGC include alternate splicing, proteolysis and N-glycosylation. Smgc is localized on rat chromosome 7q34-35 and on mouse chromosome 15E3, both immediately upstream of the high molecular weight salivary mucin, Muc19. Amino acid sequence identity between the signal peptides of SMGC, human MUC19 and pig submaxillary mucin suggest that rat and mouse Smgc and Muc19 arose from a single ancestral mucin gene.