A full-length cDNA clone for a novel glutathione S-transferase was isolated from Arabidopsis thaliana and characterized. The cDNA encodes a polypeptide of 218 amino acids with a calculated molecular mass of 24,363 Da. The sequence was most related to the theta class within the glutathione-S-transferase superfamily of enzymes. The protein encoded by the cDNA was functionally expressed and enzymically active in Escherichia coli; glutathione-S-transferase activity with the standard enzyme substrate 1-chloro-2,4-dinitrobenzene was demonstrated (apparent Km, 10 mM; apparent Km for glutathione, 0.08 mM). The enzyme is substrate specific and did not use several electrophilic reduced-glutathione acceptor molecules for conjugation. However, it efficiently catalyzed the conversion of 13-hydroperoxy-9,11,15-octadecatrienoic acid (Km, 0.67 mM) as well as 13-hydroperoxy-9,11-octadecadienoic acid (Km, 0.79 mM) to the corresponding hydroxy derivatives with concomitant formation of oxidized glutathione. The enzyme did not use H2O2 as substrate. Thus, the cloned A. thaliana enzyme functions as glutathione peroxidase and, in the plant cell, may be involved in the removal of reactive organic hydroperoxides, such as the products of lipid peroxidation. The enzyme is structurally and enzymatically, however, unrelated to the selenium-containing glutathione peroxidases. Enzymic and immunoblotting data suggest that the A. thaliana enzyme is soluble and constitutively expressed in vegetative rosettes, but is under developmental control during the transition to bolting and flowering.