In an attempt to clarify the possible radio-protective action of reduced glutathione on the cellular level following total or fractionaged whole body X-irradiation, GSH was injected intraperitoneally to albino mouse prior to the X-irradiation. In the groups where X-irradiation was done, X-ray dose was set to 1,000r, and single dose was divided into 5 times, and 200 r of X-ray was given daily for 5 days. Where GSH was used, the total amount of GSH infected into each mouse was 50mg, and the 50mg of GSh was used either as a single dose or fractionated dose of 10mg daily for 5 days. RESULTS : 1. Levels of NP-SH in the XT group were higher than in the XF group, and the highest value was noted at 30 min. 2. Injection of GSH alone produced the highest value of NP-SH at 30 min., and the recovery to the normal lecels were observed soon thereafter. 3. Levels of NP-SH was found generally to be higher in the GSH+XT group comparing to the GSH+XF group, but no significant difference were noted between two groups. 4. Higher values of NP-SH were observed in the GSH+XT and GSH+XF groups than in the XT or XF groups.