Vitamin A injected into white mice stimulates cell division. The epithelium of the intestines is the most sensitive to the vitamin. Vitamin A deficiency does not lead to any considerable changes in the number of mitoses in the tissues but the regulation of cell division is strongly disturbed. In cases of avitaminosis even such a powerful stimulator of mitosis as thyroidin fails to increase mitotic activity. The presence of vitamin A in the tissues is indispensable for the stimulative effect of thyroidin on cell division. Experiments on mice with Kroker's sarkoma demonstrated that vitamin A does not change the intensity of turnor cell division, since in turnors that process is out of control of the neurohumoral system which regulates the mitotic activity in normal tissues.