Glutathione (GSH) kit utilizes a colorimetric substrate that reacts with the free thiol group on GSH to yield a highly colored product. All reagents are in solution and require simple dilution for use in the assay. Free GSH concentration in the sample is calculated from the difference between the total GSH determined and the GSH generated from GSSG. The concentration of GSH can be determined either as an endpoint read of the color developed at 405nm after 30 minutes or by measuring the rate of color development at 405nm.