As has been known for several years, thoroughly purified ribosomes contain a firmly bound serine proteinase with an optimum of activity at neutral pH. The present paper shows that the activity is found in free cytoplasmic ribosomes as well as in ribosomes detached from the membranes of the endoplasmic reticulum of rat liver. After ribosome dissociation, the proteinase activity is found only on the 40 S subunits. Recovery of the proteinase in the proteins of whole ribosomes or of 40 S subunits amounts to 44 and 65%, respectively. Ribosomes purified both from plant (Euglena) and bacterial (Acinetobacter) cells contain a serine proteinase having an activity quite comparable to that of rat liver ribosomes. In view of the recommendations of BARRETT et al. ( in REICH, RIFKIN and SHAW (eds).: Proteinases and Biological Control, Cold Spring Harbour Lab., 1975, p. 481), who no longer restrict the name "cathepsin" to acid or even lysosomal proteinases, we propose the name " ccathepsin R" for this ribosomal serine proteinase.