An acinar cell carcinoma of the pancreas, which was induced in a male Wistar rat by repeated injections of azaserine, was propagated in cell culture. Sheets of epithelial-like cells grew within 2 weeks and were subcultured serially. Initially acinar cell carcinoma in the culture medium produced a high level of amylase, but the secretion ceased rapidly as cells began to proliferate. Only negligible amounts of trypsinogen and chymotrypsinogen were detected in cell homogenates at passages 7 and 9. The chromosome distribution ranged from hypodiploid to hypertetraploid. When cultured cells were transplanted s.c. into nude mice, palpable tumors appeared within 4 weeks and could be transplanted serially. Histological examination of the tumor showed poorly differentiated carcinoma without acinar structures. Tumor homogenate contained amylase, trypsinogen, and chymotrypsinogen, and the electron microscopic examination revealed that many tumor cells contained zymogen-like granules. These results indicate that pancreatic acinar cell carcinoma in cell cultures, in which there was no differentiated function, can be activated to synthesize tissue-specific enzymes when transplanted into nude mice by yet undefined factors present in the host animals. The cell line and transplantable tumors of pancreatic acinar cell carcinoma may be useful in the analysis of the biological behavior of this type of tumor and in the study of the control mechanisms of the synthesis of tissue-specific products in cells.