1. 1. The distribution and proportion of 5-S RNA in erythroid cell ribosomes were studied by Sephadex-gel filtration and polyacrylamide-gel electrophoresis in an attempt to find some correlation between 5-S RNA content of ribosomes and cell maturation. 2. 2. Ribosomes were isolated from phenylhydrazine-induced reticulocytes. Both free and membrane-bound ribosomes were examined. These ribosomes all contained 5-S RNA as a constant proportion of the total rRNA. Ribosomes isolated from normal circulating erythroid cells and reticulocytes of varying ages also contained the same proportion of 5-S RNA. 3. 3. It is concluded that one cannot distinguish between ribosomes from cells of varying age and presumably of different functional capabilities on the basis of gross 5-S RNA content. © 1969.