HeLa human carcinoma cells contain a high molecular mass (Mr approximately 700,000) proteinase that is identical with the ubiquitous eukaryotic 19 S cylindrical "prosome" particle. Prosomes constitute 0.6% of cell bulk protein as determined by immunochemical analysis. The prosome proteinase is not a heat shock protein, nor is it induced by cell crowding in culture or by serum deprivation. Pulse-chase experiments showed that the proteinase is relatively stable in the cells (t 1/2 approximately 5 days).