In order to investigate the effects of glutathione against ceroidosis on flounder, Paralichthys olivaceus, a serial experiment was carried out at indoor tanks in Cheju province, the southern island of R.O.Korea, from October 1 to November 4 of 1991. The choosed group of flounder for this experiment, ranged from 92 to 122g in body weight, already showed a distinct histological ceroidosis in liver, spleen, kidney about six months before this experiment. They were fed on formulated diets supplemented with glutathione of 0mg, 1mg, 2mg, 5mg, 10mg per kg of diet per day. Some flounders were fed on the decomposed diet as a control continuosuly. Two individuals fed on the experimental diets were cought at an interval 7 days for 35 days and analyzed the values of blood indexs. 1. While the values of RBC, Hb, Ht, MCHC, MCV of flounder fed on the decomposed diet and the glutathione non-supplemented diet were not changed compared with the beginning levels of experiment, those fed on the glutathione supplemented diets, showed markedly increased values from 7th day of experiment and from 21th day of experiment reached to the values of healthy flounder. 2. The variation of TP, Glu, Alp, BUN, TG, T-cho, GOT and GPT were not observed in the groups fed on the control diet and glutathione non-supplemented diet. In contrast, those values in the groups fed on the glutathione supplemented diets were markedly decreased from 7th day and become constant from 21th day. 3. The effect of glutathione supplementation with high concentration in diets showed much better improvements of symptoms concequently, compared with those of low concentration diets.