The 4-hydroxyalk-2-enals are established products of lipid peroxidation that are conjugated with intracellular glutathione. Cytosolic glutathione transferases from rat liver were shown to give high specific activities with 4-hydroxynonenal and 4-hydroxydecenal. The isoenzyme giving the highest specific activity was glutathione transferase 4-4. The rate of the spontaneous conjugation reaction is negligible in comparison with the rate calculated for the cellular concentration of the glutathione transferases. It is proposed that a major biological function of the glutathione transferases is to protect the cell against products of oxidative metabolism, such as epoxides, organic hydroperoxides, and 4-hydroxyalkenals.