Three proteolytic enzymes with different specificities produced essentially the same degradation pattern of the proteins of ribonucleoprotein particles. Proteins with apparent molecular weights of 144 000, 133 000 and 115 000 were hydrolyzed most readily which may indicate an exterior location in the ribonucleoprotein complex. The major particle proteins in the range of 30 000-42 000 daltons were much less susceptible to proteinases than the high molecular weight species. Proteins of 31 000, 33 000 and 42 000 daltons were resistant to proteolysis. The degradation rates of the proteins in intact ribonucleoprotein particles were considerably lower than rates obtained with the same proteins separated from the RNA. Incubation with trypsin and recentrifugation of polyparticles led to a shift in the sedimentation constant from 90 S to 0-20 S, corresponding to a decrease in the protein to RNA ratio from 4 to 1.3. No significant change in the small molecular weight RNA pattern was detected after trypsin digestion.