Proteasomes (multicatalytic proteinases) have been purified from spermatozoa and eggs of sea urchins by successive chromatographies on DEAE-cellulose, hydroxylapatite, and Sepharose 6B. 2. The isolated sperm and egg proteasomes showed similar properties such as molecular weight (both 630,000) and multiple catalytic activity (pH optimum, substrate specificity, inhibitor-susceptibility and SDS-stimulation). 3. Distinct protein components were detected between sperm and egg proteasomes by SDS-polyacrylamide gel electrophoresis and immunoblot analysis using antibody against egg proteasome.