Abstract
Cycloheximide is used to study the entrance of messenger RNA into membrane-bound and into free polyribosomes. The drug has no effect on the transport of messenger RNA from nucleus to cytoplasm. It inhibits by 35 to 50% the attachment of newly-synthesized messenger RNA to membranes. It increases the concentration of newly-synthesized messenger RNA in small polyribosome structures. These structures are present on free polyribosomes and absent on membrane-bound polyribosomes. They are shown to be precursors of both large membrane-bound and of large free polyribosomes.