Proteasomes (large multicatalytic proteinase complexes) are abundant in rat liver, constituting approximately 1.0% of the total soluble proteins. In the present study, the apparent half-life of the proteasomes was determined to be 12-15 days from the decay curve of isotopically labeled enzymes in vivo, suggesting their slow turnover. This finding together with the ubiquitous distribution of proteasomes in eukaryotic cells (Tanaka et al., 1988 J. Biol. Chem. 263, 16209-16217) indicates that proteasomes belong to a family of proteins with house-keeping function.