Lipid peroxides (LPO) are helpful in studying peroxidation levels in biological materials, although the levels might reflect only the physiological conditions just now. In the living body, it is considered that LPOs and related compounds are decomposed to more inactive compounds shortly after their formation and/or reduced to more stable hydroxy compounds by enzymes such as glutathione peroxidase (GPx). Thus, the levels of hydroxy lipids (L-OH) rather than LPOs may provide more important information concerning oxidative condition of the living body. We studied the levels of L-OH of liver in live fish. The levels of hydroxy lipids were noted to be higher in the order of diseased fish, cultured fish and wild fish, reflecting the strength of oxidative stress. Additionally, in order to investigate the relationships between levels of LPO, L-OH and of antioxidative enzymes in live fish, cultured cells from tilapia liver were treated with oxidative stressors. These results suggest that the elevation of the L-OH level is closely associated with the elevation of GPx activity. Consequently, oxidative status of live fish was shown clearly for the first time using new indices of L-OH. L-OH levels may be a good index for the integrated level of oxidative stress in live fish.