Abstract
Heterogeneity among the free small ribosomal subunits of rat liver was studied. Newly made free small subunits differ from the bulk of the free subunits in the following ways. (1) The newly made free small subunits sediment slightly faster than most free small subunits and form a band at slightly lower densities. (2) Mature free small subunits, prepared under conditions which minimize the amount of loosely bound protein, readily dimerize in solutions of low ionic strength. Little dimerization is found with newly formed subunits. This permits the separation of fractions enriched in newly formed subunits. (3) The fraction enriched in newly formed subunits contains a distinctive additional protein as compared the most of the free small subunits.