All factors required for protein synthesis are retained on heparin bound to Sepharose.

1. Postmitochondrial supernatants of rabbit reticulocyte lysates were chromatographed on heparin bound to Sepharose 4B, and the fraction retained on affinity columns was separated by subsequent gel filtration on Sepharose 4B into three fractions, two of them active in protein synthesis. 2. The heavier fraction sedimented at 40S and contained more than 10% RNA. This consisted predominantly of a 12S component, with smaller amounts of the 9S and 4S RNA species. The lighter fraction (18-20S) was composed of proteins with less than 1% RNA. 3. Different enzymic activities were associated with these fractions. 4. In the presence of both fractions, efficient translation took place on combined ribosomal subunits of rat liver with added cofactors. Globin messenger ribonucleoprotein stimulated this translation 5-6-fold. 5. Relatively large complexes of all factors required for protein synthesis are apparently isolated from reticulocytes by affinity chromatography on heparin-Sepharose 4B. Such complexes may occur naturally in the cytoplasm of mammalian cells.