The effects of the OX substance, which was extracted from the liver of irradiated rabbits, on the sperm and egg, unfertilized and fertilized eggs of sea urchin (Hemicentrotus pulcherrimus) have been observed. 1. The sperms exposed to OX showed a decreased mortility and lost the ability to stimul to the egg to raise up fertilizing membrane suggesting the inability of giving physical shock to the egg surface or the damage of acrosome. 2. The unfertilized eggs exposed to OX retain the ability of the formation of fertiliging membrane and cleavage but showed remarkable delay in both the length of time for the membrane formation and cleavage. 3. The most sensitive effects on the fertilized eggs have been observed on the initial stage of fusion of sperm and egg nuclei and the metaphase. Immediately after the fertilization in the egg exposed to OX the fusion of sperm and egg nuclei was inhibited and in the cells on metaphase the cleavage was delayed by the prolonged period of transition from metaphase to telophase. 4. From the results with the consideration of the effects of X-rays on cleavage appearing in the past report the authors concluded the OX will be the essential substance representing the effects of X-rays on the cell cleavage.