The nature of the growth-stimulating effect of glucosylceramide was studied. Mice were injected intraperitoneally with emulsified glucosylceramide and conduritol B epoxide, an inhibitor of cerebroside glucosidase. Within one or two days, the liver grew 18-24%, as reported. Two enzymes involved in DNA synthesis also increased more than the weight. The total liver activity of thymidine kinase increased 46-73%, and the total activity of ornithine decarboxylase increased as much as 101%. It is suggested that elevated liver levels of glucocerebroside stimulate cell proliferation through a relatively direct mechanism.