An RT-PCR-derived clone encoding a stress-inducible glutathione transferase (GSTGm1) from soybean has been overexpressed in E. coli. The enzyme was active as the dimer GSTGm1-1 and showed GST and glutathione peroxidase activity toward diverse xenobiotics, including analogues of natural stress-metabolites. The selective herbicides, fomesafen and acifluorfen, were conjugated more actively with homoglutathione (hGSH), the major thiol in soybean, than with glutathione (GSH). No thiol preference was shown with the related herbicide, fluorodifen, while GSH was preferred with metolachlor and most non-herbicide substrates. Similar thiol-dependent specificities were observed in GST preparations from plants of varying GSH/hGSH content.