The X-ray crystal structure of inactivated carboxypeptidase A by (2R,3S)-2-benzyl-3,4-epoxybutanoic acid, a pseudomechanism-based inactivator, was determined to show that the carboxylate of Glu-270 at the active site of the enzyme is covalently modified : the inhibitor is tethered to the carboxylate forming an ester linkage which is brought about by the attack of the carboxylate on the oxirane ring of the inhibitor. Examination of the crystal structure in comparison with that inactivated by its enantiomer, (2S,3R)-2-benzyl-3,4-epoxybutanoic acid, shows that the two inhibitors are bound covalently to the enzyme in a symmetrical fashion. An explanation for the lack of inactivating activity found previously with(2R,3R)- as well as (2S,3S)-2-benzyl-3,4-epoxybutanoic acids was offered.