The known ability of suspensions of unfertilized sea urchin eggs to incorporate amino acids into protein has been examined in four kinds of experiments with Lytechinus pictus. These show: (1) that the incorporation is not attributable to bacteria or other contaminating cells in the suspension but to the eggs themselves; (2) that it is inhibitable by puromycin approximately to the same extent as is protein synthesis in the fertilized eggs; (3) that it occurs upon polysomes; (4) that it is a property of all the eggs and not just a small proportion of "unripe" ones in the suspension. It is concluded that the ripe unfertilized egg of sea urchins is not synthetically inert but engages in true protein synthesis although at a much lower rate than during oogenesis or after fertilization.