Glutathione S-transferase cGSTM1-1, an avian class-mu enzyme with high sequence identity with rGSTM3-3, was expressed heterologously in Escherichia coli. The three-dimensional structure of this protein that co-crystallized with an inhibitor, S-hexylglutathione, was determined by the molecular replacement method and refined to 1.94 A resolution. The three-dimensional structure and the folding topology of the dimeric cGSTM1-1 closely resembles those of other class-mu GSTs. The bound inhibitor, S-hexylglutathione, orients in disparate directions in the two subunits. The combined space occupied by the hexyl moiety of the inhibitors overlaps with that reported for rGSTM1-1 co-crystallized with (9 S,10 S)-9-(S-glutathionyl)-10-hydroxy-9,10-dihydrophenanthrene. Conformational differences at a flexible loop (residue 35 to 40) were also observed between the crystal structures of cGSTM1-1 and rGSTM1-1.cGSTM1-1 has the highest epoxidase activity among all the class-mu enzymes reported. Tyr115, has been identified as a residue that participates in the epoxidase activity of class-mu glutathione S-transferase and is conserved in cGSTM1-1. The epoxidase and trans-4-phenyl-3-buten-2-one conjugating activity of cGSTM1-1 are decreased drastically but not abolished by replacing Tyr115 with phenylalanine. The specificity constant of the cGSTM1-1(Y115F) mutant, with 1-chloro-2,4-dinitrobenzene as substrate, is 15-fold higher than that of the wild-type enzyme.