The effect of unsaturted fatty acid fraction from the liver of irradiated animal on fertilization and cell division of sea urchin egg (Hemicentrotas tulcherrimus) have been observed. The sperms exposed to the substance for 5 minutes decrease in their mortility and lost the fertilization acivitity resuting in the lose of their ability for the formation of fertilizing membrae in eggs. The eggs exposed to this substance after fertilization present the stop or the delay in their cleavage. Such an action on eggs becomes marked when the concentration of this substance increases. The most susceptible stage seems to be the metaphase at which the chromosoms are arrested on the equatolial plate loosing the tendency to move toward the poles. These effects of this substace can be seen even after removing the peroxide as equally in the original substance.