Using dimyristoyl L-alpha-phosphatidylcholine as a substrate, the effect of coenzyme Q10 on phospholipid digestion by phospholipase A2 and phospholipase C was investigated. Free myristic acids released by phospholipase A2, and myristic acids of dimyristoyl glyceride released by phospholipase C were methylated and determined quantitatively by gas-chromatography. Phospholipase A2 or phospholipase C released myristic acids dose-dependently from the substrate. Coenzyme Q10 prevented dose-dependently the hydrolysis of the substrate caused by phospholipase. These results suggest that pharmacological action of coenzyme Q10 could be attributed to its protection of membrane phospholipids against the attack of phospholipases.