GLUTATHIONE REDUCTASE (GR) reduces oxidized glutathione (GSSG) to the reduced sulfhydryl form GSH which is an important cellular antioxidant. A high GSH/GSSG ratio is important for protection against oxidative stress. Thus, measurement of GR activity is used as indicator for oxidative stress. This non-radioactive, colorimetric GR assay is designed to accurately measure GR activity in biological samples with a Method that utilizes Ellman's Method in which DTNB reacts with the GSH generated from the reduction of GSSG by the GR in a sample to form a yellow product (TNB2-). The rate of change in the optical density, measured at 412 nm, is directly proportional to GR activity in the sample.