Molecular weights of ribosomes and ribosomal subunits prepared by precipitation in (NH 4) 2SO 4, by washing in NH 4Cl and with no wash, were compared. The former two methods apparently removed approximately 100,000 daltons of material from each of the subunits, as compared with those prepared with no wash. Physical characterization of the 70 s ribosomes and the 50 s subunits in 0.5 m-NH 4Cl showed these particles to have sedimentation coefficients of 65 s and 45 s respectively in this medium. Sedimentation equilibrium studies showed the 65 s and 45 s particles to have essentially the same weight as the 70 s and 50 s particles. Small-angle X-ray scattering and viscosity studies indicated that the particles were not unfolded in 0.5 m-NH 4Cl. However, a significant decrease in the density increment ( ∂ϱ ∂c ) resulted from placing the particles in this medium. This decrease is in part responsible for the decrease in the sedimentation coefficient and in the zero-angle X-ray scattering. When 45 s particles are placed in solutions of very low magnesium concentration the sedimentation coefficient decreases further to 39 s. Small-angle X-ray scattering studies show that some unfolding has occured in this latter transition.