Effects of phospholipids, their metabolites and endogenous sperm lipids on the chymotrypsin-like activity of proteasome purified from sperm of the sea urchin, Strongylocentrotus intermedius were examined. Some lysophospholipids remarkably enhanced the activity. The most prominent activation was found in lysophosphatidylinositol (LPI) which enhanced about 12-fold at 2.5 μg/ml. On the other hand, higher concentrations (above 250 μg/ml) were required for the enhancement of the activity by some saturated fatty acids and phospholipids. Lipids extracted from sperm also were effective in the enhancement, and those from sperm which were treated for 15 sec in egg jelly were more effective than those from untreated sperm. These results suggest that certain metabolites belonging to lysophospholipids are produced during the acrosome reaction and activate sperm proteasome. Also, they are not inconsistent with our view that the chymotrypsin-like activity of sperm proteasome participates in the acrosome reaction (23, 24).