A series of cholesterol hydroperoxides has been prepared and tested as inactivators of calmodulin. Two previously undescribed compounds, tentatively identified as 20-(R)-25-dihydroperoxy-5-cholesten-3 beta-ol and its 20-(S) isomer inactivate calmodulin with 50% loss of activity at 5-10 microM. Cholesterol derivatives with a single hydroperoxy group at C-20 or C-25 are less effective, while 7 alpha-hydroperoxy-cholesterol and 25-hydroxy-cholesterol are inactive. The side-chain hydroperoxide compounds were isolated from a mixture shown earlier to suppress formation of fatty streaks in aortas of rabbits fed a diet supplemented with cholesterol.