Reduced glutathione was found to guard against photodecomposition (under fluorescent light) of the anticancer drug dacarbazine (DTIC). Increase in pH appeared to accelerate the degradation of DTIC within a pH range of 4.5-8.1. The stabilizing effect of reduced glutathione was found to increase with its concentration reaching an optimum value at 5 mg%. Increase of ionic strength slightly enhanced the photostability of DTIC solutions in pH 7 phosphate buffer (0.05 M). Different buffer systems at pH 4.5 did not significantly alter the degradation rate of the drug in the absence of reduced glutathione. However, in presence of reduced glutathione, dacarbazine appeared to be less stable in acetate buffer of pH 4.5.