We have previously established that secretory proteins from a rat acinar cell tumor lack two forms of procarboxypeptidase B, are deficient in a major lipase species, and possess markedly reduced amounts of the basic proteins proelastase, basic chymotrypsinogen, basic trypsinogen and ribonuclease (Iwanij, V., and J. D. Jamieson, J. Cell Biol., 95:734-741). Because secretory proteins are markers for acinar cell differentiation, we sought to establish whether the secretory protein profile of the acinar cell tumor is unique to the transformed cell or whether it resembles that of a stage of normal pancreatic development. To this end, we compared the secretory protein pattern from acinar tumor cells with that of rat pancreatic rudiments at days 19-22 of gestation and through day 21 of the postnatal period. Twodimensional IEF-SDS gel electrophoresis coupled with biosynthetic labeling and fluorography indicates a time-dependent appearance of individual secretory proteins with basic polypeptides, except for amylase, appearing in the terminal stages of differentiation. In comparison, the secretory protein pattern of the acinar tumor cells most closely resembles that of day-19 embryonic pancreatic rudiments. We propose that the cells of the acinar cell tumor may, in part, mirror a stage of normal pancreatic development.