The action of the cytoplasmic and nuclear fractions of Ehrlich's ascites carcinoma cells of mice on cell division in the tumor and its dependence on adrenalin were studied. The cytoplasmic fraction was shown to contain a tissue-specific inhibitor of mitoses. The nuclear fraction was found to contain a stimulator of cell division whose effect is not tissue-specific. Inhibition and stimulation of mitotic activity by the cytoplasmic and nuclear fractions, respectively, are observed in the absence of adrenalin in the medium. However, the action of these fractions undergoes definite hormone-induced changes.