Reactive immunization was used to generate catalytic antibodies that use the enamine mechanism common to the natural class I aldolase enzymes. In order to investigate the possibility of exploiting the imine and enamine intermediates programmed into antibody catalysts by reactive immunization and the features which antibody aldolases share with naturally evolved catalysts, we have studied their ability to catalyze the decarboxylation of structurally related beta-keto acids. Both aldolase antibodies were shown to efficiently catalyze the decarboxylation of two hapten-related beta-keto acids with rate enhancements (k(cat)/k(uncat)) between 4959 and 14774. Inhibition studies support the role of an essential lysine residue in the active site of the antibodies and the formation of a cyanide accessible imine intermediate in the mechanism. Investigation of the decarboxylation reaction of 2-{3’-(4 "-acetamidophenyl)propyl}-acetoacetic acid, 4, to 6-(4’-acetamidophenyl)-2-hexanone, 12, in the presence of O-18-labeled water by electrospray mass spectrometry revealed obligatory incorporation of O-18 in the antibody-catalyzed reaction consistent with decarboxylation proceeding via an imine intermediate. These studies demonstrate that reactive immunization may be utilized to program in fine detail the mechanism of catalytic antibodies and our ability to exploit the programmed reaction coordinate for different catalytic tasks.