20 S Proteasomes are large proteinase complexes found in eukaryotic cells where they degrade cell proteins in an ATP-dependent manner. Proteasomes consist of 14 different subunits. One of them, zeta, was found in HeLa cells at a concentration of 890 microg per g of cell protein. A large proportion of zeta was found in the free state rather than incorporated into proteasomes, namely 28% in HeLa cells and 37% in BSC-1 cells. Free zeta was found in both nuclei and cytoplasm. In HeLa cells free zeta had a t1/2 of 2.8 h, compared to 5 d for proteasomes, and did not exchange with zeta in proteasomes. We confirmed (Petit F et al.: Biochem. J. 326: 93-98 (1997)) that both 20 S proteasomes and free zeta subunits possess RNase activity though the activities were very low: 4 mMoles and 0.6 mMoles of tobacco mosaic virus RNA degraded per mole of enzyme per min, respectively. The physiological function of the relatively abundant zeta monomers is not known.