Cayman’s TrxR Assay provides a convenient method for quantifying mammalian TrxR activity from tissue homogenates and cell lysates in a colorimetric 96-well plate format. In this assay, TrxR uses NADPH to reduce DTNB to 5-thio-2-nitrobenzoic acid (TNB) which absorbs strongly at 405-414 nm. Measurement of TrxR activity in the absence and in the presence of aurothiomalate, a specific TrxR inhibitor included in the kit, allows for correction of non-thioredoxin reductase-independent DTNB reduction.