The three-dimensional crystal structure of pi class glutathione S-transferase YfYf from mouse liver complexed with the inhibitor S-(p-nitrobenzyl)glutathione has been determined at 1.8 A resolution by X-ray diffraction. In addition two complexes with glutathione sulphonic acid and S-hexylglutathione have been determined at resolutions of 1.9 and 2.2 A, respectively. The high resolution of the S-(p-nitrobenzyl)glutathione complex allows a detailed analysis of the active site including the hydrophobic (H-) subsite. The nitrobenzyl moiety occupies a hydrophobic pocket with its aromatic ring sandwiched between Phe8 and the hydroxyl group of Tyr108. An insertion of two residues Gly41 and Leu42, with respect to the pig enzyme, splits helix alpha B into an alpha-helix and a 3(10) helix. Water bridges between carbonyl oxygen atoms of the alpha-helix at its C terminus and the amide NH groups of the 3(10) helix at its N terminus provide structural continuity between these two secondary elements. Tyr7 appears to be the only residue close to the sulphur atom of glutathione, while three conserved water molecules lie in the surrounding area in all complexes. The enzyme mechanism is discussed on the basis of the structural analysis.