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The work with Ehrlich ascites tumor cells presented in this report shows that all the smaller (40 S) ribosomal subunits found free in the cytoplasm are combined with substantial amounts of exchangeable, nonribosomal protein. This combination represents a specific high affinity interaction, for the subunit-associated protein, after release by 0.5 m KCl, will recombine quantitatively with the subunit when the salt concentration is lowered, and this recombination occurs even in the presence of a large excess of cytoplasmic protein. The amount of the subunit-associated protein is sufficient to permit separating the free (native) 40 S subunits on the basis of their buoyant density from those derived by dissociating ribosomes. In CsCl equilibrium density gradients the native particles appear as two heterodisperse bands centered at densities 1.49 and 1.40 g per cm3, whereas the derived subunits form a single narrow band at 1.51 g per cm3. Calculations from these values, confirmed by direct measurement, indicate that the 1.49 and 1.40 native particles contain about 1 x 105 and 7 x 105 daltons of associated protein, respectively. During alterations in rate of protein synthesis, significant changes in numbers of native subunits occur, although these are small compared with the many-fold fluctuations in monomeric ribosomes that one observes. When peptide chain initiation is slowed relative to chain completion and ribosome runoff, the number of density 1.49 subunits increases immediately, without appreciable change in the 1.40 class, and approaches a maximum when the monomer fraction is just beginning to rise. This early increase in the 1.49 subunits is consistent with the concept that direct coupling of the larger and smaller subunits following runoff is blocked by combination of the smaller subunits with protein factor (or factors) and that monomers accumulate when the limited supply of free factor (or factors) has all been bound. The subunit-associated proteins have not yet been isolated and characterized. However, acrylamide gel electrophoresis of purified native 40 S particles reveals a small number of prominent bands in the molecular weight range 50,000 to 200,000 (all bands of the derived subunit being less than 50,000).