Using sea-urchin eggs, Euplotes woodruffi, HeLa cells, and Ehrlich ascites tumor cells, the author studied the action of OX substance on the DNA and protein synthesis of these cells, and obtained the following results. 1. OX substance has been found to act as to impede the incorporation of (32)P to DNA in Ehrlich ascites tumor cells. 2. OX substance leads the nuclei of Euplotes, HeLa and Ehrlich ascites tumor cells into a picnotic state. 3. OX substance is found to possess an action to diminish the DNA contents in the nuclei of HeLa and Ehrlich ascites tumor cells while it tends to increase transiently the DNA content in the nucleus of sea-urchin egg but later decreases it. 4. It has been found that OX substance brings about the disturbance on the mitotic apparatus or the functional disturbance in Ehrlich ascites tumor cells and sea-urchin eggs. 5. In addition, this substance acts as to inhibit the protein synthesis of Ehrlich ascites tumor cells. 6. From these findings, it seems that OX acts temporarily on the mitotic apparatus, thus interfering with the cell division as well as giving rise to the degeneration of the cells. accompanied by the secondary inhibition of the synthesis of DNA and RNA in the cells.