Lipid free-radical oxidation has been studied in vivo in the mitochondrial fractions of the liver of rats fed RoseOxW (camosic acid nutritional supplement) by measuring chemiluminescence. The kinetics of the lipid chemiluminescence in rats fed RoseOx@ are significantly different from those of the control. The intensity of the chemiluminescence fast flash decreases by 45% (p < 0.01 ), which indicates a reduction of lipid peroxides. The time between fast and slow flashes increases by 96% (p < 0.05), which indicates a higher content of antioxidants in the lipid membrane. The in vitro experiments in rat liver mitochondrial fraction display more effective antioxidant action of ol-tocopherol in 1 PM concentration than 1 p,M camosic acid by an increase of the time between fast and slow chemiluminescence flashes (p < 0.01). However, the higher antioxidant activity of 1 p,M camosic acid by a decrease of intensity of the chemiluminescence fast (p < 0.05) and slow (p < 0.05) flashes in comparison with a-tocopherol is revealed in these experimental conditions in vitro. Camosic acid has antioxidant effects on homogeneous oxidation in vitro as well. The chemiluminescence of methyl oleate initiated by 2,2'-azobis( Z-methylpropionitrile) decreases by 25% (p < 0.01) in the presence of 13.5 p,M camosic acid. 13.5 FM cY-tocopherol decreases the methyl oleate chemiluminescence by 45%. A higher antioxidant activity of cr-tocopherol in comparison with camosic acid (p < 0.001) is found in this system. These results indicate that RoseOxm reduces free-radical-induced lipid peroxidation in vivo. In vitro data show that camosic acid has direct action as an antioxidant, rather than as a membrane-structure modifier.