BY comparison with those of prokaryotic cells, especially Escherichia coli, little is known about the ribosornes of eukaryotes. The present analysis of ribosomal proteins from rabbit reticulocytes indicates that—in spite of the larger size of the particle and a higher protein content—the number of different proteins is not significantly greater than that in E. coli ribosomes. There may be multiple copies of some proteins and other proteins may be present in only a fraction of the ribosomes. Proteins from both ribosomal subunits are evidently considerably more basic than the corresponding proteins from E. coli subunits.