It has been shown (Maggio et al., 1964) that RNA extracted from unfertilized sea urchip eggs stimulates incorporation of amino acids by ribosomes from rat liver or from sea urchin embryo. Ribosomes of unfertilized eggs are not stimulated. Following treatment with trypsin, ribosomes of unfertilized eggs (1) are stimulated by RNA; (2) acquire the ability to incorporate amino acids in the absence of any exogenous RNA or polynucleotide. When trypsin treatment is followed by incubation of the ribosomes in their own postribosomal supernatant, the endogenous incorporation is greatly increased. It is suggested that in the unfertilized eggs ribosomes carrying attached molecules of messenger RNA are present which, however, are rendered inactive by a protein coat. Activation of protein synthesis, upon fertilization would result from the removal of this protein coat due to the action of proteases that (as shown by Lundblad, 1949) become active immediately following fertilization.