Carp liver contains a glutathione S‐transferase isoenzyme which uses as best substrates l‐chloro‐2,4‐dinitrobenzene (CDNB) and cumene hydroperoxide, thus showing selenium‐independent glutathione peroxidase activity. This isoenzyme accounts for about 15% of the total activity with CDNB and does not bind to the affinity matrix of hexyl‐S‐glutathione Sepharose 6B. It has been partially purified by ionic exchange and gel filtration chromatographies and isoelectric focusing. The purified enzyme is an acidic protein of 55 kDa of relative molecular mass and has an isoelectric point at pH 5.4. Values of Km have been measured for both CDNB and reduced glutathione (GSH) substrates. The best substrates are CDNB and cumene hydroperoxide, followed by ethacrynic acid and 1,2‐epoxy‐3‐(p‐nitrophenoxy)‐propane. Hydrogen peroxide and 1,2‐dichloro‐4‐nitrobenzene are not substrates and trans‐4‐phenyl‐3‐buten‐2‐one is a very poor one. Among several glutathione S‐transferase inhibitors used, cibacron blue and rose bengal are the strongest; the herbicides, 5‐amino‐4‐chloro‐2‐phenyl‐3(2H)‐pyridazinone (Pyrazon), 2,4‐dichlorophe‐noxyacetic acid (2,4‐D), and 2‐(2‐methyl‐4‐chlorophenoxy) pro‐pionic acid (MCPA) are less effective.