2'5'-Oligoadenylates are potent protein synthesis inhibitors: they are synthesized by a polymerase which was first described in interferon-treated cells. This system may also be involved in a normal process of cell proliferation and differentiation, in the absence of any viral infection. The small intestine enterocyte has been investigated as a model to test this hypothesis. The presence of 2'5'-oligoadenylate polymerase activity is demonstrated in the intestinal mucosa of normal adult rats. The distribution of this enzyme, and of the enzymes degrading 2'5' oligoadenylates have been investigated on enterocyte pools selectively extracted, under mild conditions, from the different parts of the rat small intestine. Similar results were obtained with enterocytes extracted from the mucosa of germ-free animals.