Chemiluminescence is being increasingly employed for evaluating oxidative stress in biological systems. Using this technique, we examined the oxidative stress in trout erythrocytes. Hemolysates and erythrocyte ghosts were studied in whole and in density-separated trout erythrocytes, after exposure to superoxide anion generated by the xanthine/xanthine oxidase system. The results show varying chemiluminescence response in the studied fractions, possibly reflecting different degrees of oxidation.