As some authors suspect that ozone influences the metabolic process of fat, we tried to analyse the influence of an ozone-oxygen gas which was applied parenterally. 40 patients with gynecological cancer received 10 ml ozone-oxygen gas with a content of 450 gamma ozone and venous blood was removed before and 10 minutes after application. The serum was lyophilized and the level of fatty acids and triglycerids was determined by the method of Randerath (1965). a statistically significant decrease of the concentrations was observed after application of ozone. Different theories as to the cause of this action are discussed.