An extract was prepared from rabbit reticulocyte ribosomes after treatment with potassium chloride as described previously (Miller, Hamada, Yang, Cohen & Schweet, 1967). The participation of the extract in cell-free protein synthesis was studied. Purified polyribosomes were isolated and converted into monoribosomes by incubation in the cell-free protein-synthesis system. The monoribosomes were isolated and found to be unable to synthesize protein in the cell-free system. The addition of the ribosomal extract to the system stimulated protein synthesis. This was accompanied by the conversion of some of the monoribosomes into polyribosomes. The active component or components of the extract were shown to be protein.
