The seromucous acinar cells of the adult rat submandibular gland secrete a characteristic mucin glycoprotein and a family of unusual glutamine/glutamic acid-rich proteins (GRP). Monoclonal antibodies to the mucin and GRP localized in a very few Type III cells in glands of newborn and 1 day-old rats, using light and electron microscopic immunocytochemistry. Both mucin and GRP reactivities were present in the polymorphic Type IIIP granules during the 1st postnatal week. By 9 days after birth, the granules contained both mucin and GRP and were mucous-like in appearance. At earlier stages, however, cells containing only GRP or mucin could be found, indicating that the initiation of GRP and mucin biosynthesis may not be coordinately regulated. No reactivity was seen in the neonatal Type I cells or in duct cells at any age. Northern and Western blot analysis showed GRP mRNA and protein levels to be barely detectable at birth, with marked increases during the first 2 postnatal weeks. In contrast, Western blots of B1-immunoreactive proteins (B1-IP) showed levels highest in the 1st week and markedly decreased in the adult. Immunocytochemical colocalization, using gold particles of different sizes, showed that the B1-IP, mucin, and GRP colocalized in the granules. These results strengthen the hypothesis that the adult acinar cells develop from the neonatal Type III cells. No evidence was obtained for the involvement of Type I cells in the pathway of acinar cell development.