An aldolase antibody, 24H6, which was obtained from immunization with the large hapten 2, is shown to possess an active-site lysine residue with a perturbed pK(a) of 7.0. This antibody catalyzes both the aldol addition and the retrograde aldol fragmentation with a broad range of substrates that are structurally different from the hapten. This observation suggests that in reactive immunization with 1,3-diketones, the hapten structure governs the chemistry but not the overall organization of the active site. Hammett correlation studies of the 38C2- and 24H6-catalyzed aldol and retroaldol reactions revealed that although the two antibodies exhibit broad substrate specificities, they utilize slightly different mechanisms. While antibody 38C2 adopts a mechanism that is reminiscent of an acid-catalyzed aldol reaction, antibody 24H6 follows a mechanism that is similar to the base-catalyzed reaction.