In this paper we report the generation and mechanistic characterization of an unusually active catalytic antibody raised to a phosphonate transition-state analog of norleucine. The antibody (17E8) catalyzes the hydrolysis of both norleucine and methionine phenyl esters and is specific for enantiomers that possess the natural S configuration (L) at the alpha-carbon. The antibody shows side-chain selectivity (k(cat)/K-M) for norleucine over methionine phenyl esters and also shows selectivity for amino acid ester substrates bearing a formyl substituent at the cu-amino position. The antibody-catalyzed hydrolysis of all the ester substrates exhibits a bell-shaped pH-rate profile that is consistent with a mechanistic scheme featuring two ionizable active site residues that mediate catalysis. The calculated pK(a) values for these two residues are 8.9 and 10.1, and in the most catalytically active state of the antibody, the pK(a) 8.9 residue is deprotonated and the pK(a) 10.1 residue is protonated. In the presence of hydroxylamine, partitioning between hydrolysis and hydroxaminolysis is observed, indicating that a covalent acyl intermediate is formed in the antibody-catalyzed reaction. The hydroxaminolysis:hydrolysis product ratio increases with increasing hydroxylamine concentration, while the overall reaction velocity (as measured by phenol release) is unaffected by increasing hydroxylamine concentration. These data support a mechanism involving a covalent acyl intermediate where formation of the intermediate is the; rate-determining step in the antibody-catalyzed hydrolysis reaction.