The estimation of glutathione disulfide (GSSG) is based on the NADPH-dependent glutathione reductase reaction. A new method has been developed to eliminate the inactivating effect of N-ethylmaleimide (NEM), added to prevent glutathione oxidation, on glutathione reductase. This method takes advantage of instability of NEM in alkaline solutions. The product of NEM hydrolysis, N-ethylmaleamic acid, obtained under accurate pH-controlled conditions, is compatible with a good activity of glutathione reductase which allows total recovery and measurement of GSSG. The method, applied to estimation of GSSG content in human erythrocytes and rat liver, gives results in optimum agreement with values reported in literature. Because of its simple performance and rapidity, the procedure can be considered an improved method in removing NEM and is particularly advantageous when a large number of biological samples must be treated and estimated.