Redistribution of Mersalyl-Hg²⁰³, previously incorporated by unfertilized Paracentrotus lividus eggs, has been studied following fertilization (1-10 minutes).The radioactivities present in the eggs after the mechanical removal of the fertilization membranes at different intervals, are 60-80% of that detected in the unfertilized egg. Since the removed membranes retain only 2-7% of the total radioactivity, the difference must be ascribed to the soluble material present in the perivitelline space. According to these results it is conceivable that the sulphydryl-bearing proteins engaged in the antipolyspermic mechanism are released into the perivitelline space mostly following the rupture of the cortical bodies.