We have used homology modelling, based on the crystal structure of the human glutathione S-transferase (GST) A1-1, to obtain the three-dimensional structures of rat GSTA3 and rat GSTA5 subunits bound to S-aflatoxinyl-glutathione. The resulting models highlight two residues, at positions 208 and 108, that could be important for determining, either directly or indirectly, substrate specificity for aflatoxin-exo-8,9-epoxide among the Alpha-class GSTs. Residues at these positions were mutated in human GSTA1-1 (Met-208, Leu-108), rat GSTA3-3 (Glu-208, His-108) and rat GSTA5-5 (Asp-208, Tyr-108): in the active rat GSTA5-5 to those in the inactive GSTA1-1; and in the inactive human GSTA1-1 and rat GSTA3-3 to those in the active rat GSTA5-5. These studies show clearly that, in all three GSTs, an aspartate residue at position 208 is a prerequisite for high activity in aflatoxin-exo-8,9-epoxide conjugation, although this alone is not sufficient; other residues in the vicinity, particularly residues 103-112, are important, perhaps for the optimal orientation of the aflatoxin-exo-8,9-epoxide in the active site for catalysis to occur.