The polymerase chain reaction was used to amplify a cDNA encoding the catalytic region of ras-guanine nucleotide release factor (ras-GRF1) from mouse embryonic stem cell mRNA. Antibodies directed against this protein were prepared and affinity purified. Western immunoblotting of rat tissue lysates revealed a 140-kDa protein in brain as expected but, in addition, a strongly immunoreactive 54-kDa protein, p54, was identified in pancreas. Expression of ras-GRF1 in pancreas was confirmed at the RNA level by reverse-transcriptase-coupled polymerase chain reaction analysis; p54 may therefore correspond to a form of ras-GRF1 or a closely related protein. The cellular and subcellular localization of p54 was investigated by enzyme-linked immunocytochemistry and immunogold electron microscopy. In the pancreas, p54 was expressed primarily in acinar cells, where it was localized along the basolateral and apicolateral plasma membranes. Indirect immunofluorescence microscopy of cultured acini further indicated that the plasma membrane localization of p54 was dependent on the maintenance of the acinar histotype and organized acinar structure. When primary acinar cells were permitted to dissipate into monolayer cultures devoid of zymogen granules, ras-GRF1 staining became cytosolic. Our results suggest that ras-GRF1 is involved in the structure and function of the pancreas.