The selectivity of response by the Syrian hamster pancreas to the neoplastic effect of N-nitrosobis(2-oxopropyl)amine (BOP) was demonstrated by single subcutaneous injections of the compound. Only occasional tumors were induced simultaneously with those of the pancreas in the biliary ducts, kidneys and lungs of a few hamsters. In the pancreas, the ductules, especially those of an intrainsular location, were the cells primarily affected. The responded to doses as low as 1/40 of the LD50, whereas ductal lesions were found only in some hamsters treated with doses above 1/5 of the LD50. Hence it is concluded that hamster ductular cells are the most responsive to the carcinogenic action of BOP.