This paper describes the substrate specificity, synthetic scope, and efficiency of aldolase catalytic antibodies 38C2 and 33F12. These antibodies use the enamine mechanism common to the natural Class I aldolase enzymes. Substrates for these catalysts, 23 donors and 16 accepters, have been identified. The aldol acceptor specificity is expected to be much broader than that defined here since all aldehydes tested, with the exception polyhydroxylated aldehydes, were substrates for the antibodies. 38C2 and 33F12 have been shown to catalyze intermolecular ketone-ketone, ketone-aldehyde, aldehyde-ketone, and aldehyde-aldehyde aldol addition reactions and in some cases to catalyze their subsequent dehydration to yield aldol condensation products. Substrates for intramolecular aldol reactions have also been defined. With acetone as the aldol donor substrate a new stereogenic center is formed by attack on the si-face of the aldehyde with ee＇s in most cases exceeding 95%. With hydroxyacetone as the donor substrate, attack occurs on the re-face, generating an alpha,beta-dihydroxy ketone with two stereogenic centers of the alpha-syn configuration in 70 to >98% ee. With fluoroacetone donor reactions, the major product is a syn alpha-fluoro-beta-hydroxy ketone with 95% ee. Studies of retroaldol reactions demonstrate that the antibodies provide up to 10(8)-fold enhanced efficiency relative to simple amine-catalyzed reactions.