The Glutathione Detection Kit I provides a convenient, colorimetric method for analyzing either total glutathione or only the reduced form of glutathione using a microtiter plate reader. The assay utilizes the glutathione recycling system with DTNB and glutathione reductase: GSH is regenerated from GSSG by glutathione reductase, and will again react with DTNB to produce yellow 2-nitro-5-thiobenzoic acid that can be determined spectrophotometrically at 412 nm. This recycling reaction dramatically improves the sensitivity of total glutathione detection – the kit can quantify total glutathione from 1-100 ng/well. For detecting lower glutathione concentrations, such as in blood samples, increasing reaction time will significantly improve the signal. The kit can also specifically detect the reduced form of glutathione (GSH) by omitting the glutathione reductase from the reaction mixture.