Abstract
The membrane-bound ribosomes of HeLa cells were analyzed in CsCl buoyant-density gradients. There exist two classes: a heavy class with a density of 1.55 g/ cm
3 and a light class heterogeneously distributed with a mean density of 1.49 g/cm
3. The lower density of the light class is due to an increased ratio of protein to RNA. This additional protein is not removed with increased ionic strength. Both classes of ribosomes incorporate amino acids at a rate comparable to non-membrane-bound ribosomes. The heavy and light ribosomes appear to correspond to the EDTA-sensitive and the EDTA-resistant ribosomes, respectively.