Atherosclerotic plaques contain much cholesterol and small amounts of cholesta-3:5-dien-7-one and other unidentified congeners with traces of carotene. The dienone and cholest-4-en-3-one were not found in brain from atherosclerotic subjects or from aged rats even when unsaponifiable fractions were obtained under conditions capable of isomerizing appreciably the unconjugated poly-ene acids of brain lipids. Cholesterol gallstones do not contain detectable amounts of dienone. The evidence indicating that the dienone is not an artifact makes necessary further investigation of the metabolism of the compound.