In an attempt to clarify the possible radio-protective action of reduced glutathione (GSH) on the cellularlevel following total or fractionated whole body X-irradiation, GSH was injected intraperitoneally to albino mouseprior to the X-irradiation. Inthe groups where X-irradiation was done, X-ray dose was set to 1,000r, and singledose of 1,000 r was given in the total irradiation group (XT), whereas in the fractional X-irradiated group (XF),the dose was divided into 5 times, and 200 r of X-ray was given daily for 5 days. Where GSH was used, the totalamount of GSH injected into each mouse was 50mg, and the 50mg of GSH was used either as a single dose orfractionated dose of 10mg dailly for 5 days. Intrinsic NP-SH was measured at 30 minutes, 1,5, and 24 hours in thefollowing groups, i.e. 1) Group which was X-irradiated with 1,000 r as a single dose (XT) without prior injectionof GSH. 2) Group which was X-irradiated with 200 r each for five days without prior injection of GSH (XF), 3)Group which was injected with single dose of 50mg of GSH, but not X-irradiated (GSH) 4) Group in which single doseof 50mg of GSH was injected prior to the X-iradiation with single dose of 1,000 r (GSH+XT), 5) Group in whichfractionated dose of 10mg each of GSH was injected prior to X-irradiation with fractionated X-ray dose of 200 reach for 5 days at daily intervals (GSH+XF), and the results were compared with the group of 6, that is thenormal. The results are summarized as follows: 1. Levels of NP-SH in the XT group were higher than in the XFgroup, and the highest value was noted at 30min. 2. Injection of GSH alone produced the highest value of NP-SH at30 min, and the recovery to the normal levels were observed soon thereafter. 3. Levels of NP-SH was foundgenerally to be higher in the GSH+XT group comparing to the GSH+XF group, but no significant difference were notedbetween two groups. 4. Higher values of NP-SH were observed in the GSH+XT and GSH+XF groups than in the XT or XFgroups.