It was established that in blood of patients with ischemic heart disease due to atherosclerosis of the coronary arteries (the diagnosis was verified in selective coronaroangiography) the content of acylhydroperoxides grows while the activity of glutathione-peroxidase II decreases. In blood of patients with no damage to the coronary vessels (according to the results of angiography), glutathione-peroxidase II activity does not differ significantly from the values in the control group. The decrease of glutathione-peroxidase II activity was most marked in patients with ischemic heart disease and hypercholesterolemia. It is suggested that the increase in the peroxide content in blood of patients with ischemic heart disease may be due to the sharp decrease in the activity of glutathione-peroxidase II.