A monoclonal antibody (MAb) was produced against the p-nitrophenylphosphate derivative of 3 alpha,5 beta-lithocholic acid, a transition-state analog for hydrolysis of a steroidal p-nitrophenylcarbonate. The indicated reaction was catalyzed by this Ab with kinetic constants k(cat) = 4.0 x 10(-2)/min and K-m = 3.3 mu M at pH 9.0 and 35 degrees C. The Ab also hydrolyzed the isomeric p-nitrophenylcarbonate of 3 beta,5 beta-lithocholic acid with k(cat) = 8.4 x 10(-2)/min and K-m = 1.0 mu M. Bovine serum albumin (BSA) was found to catalyze the same reactions with similar turnover rates and Michaelis constants of 15 and 14 mu M, respectively. Although the BSA-catalyzed reaction was only weakly inhibited by the phosphate ester TSA (IC50 ca. 40 mu M), the Ab-catalyzed reaction was completely inhibited at less than 1 mu M Of the TSA. The relative rates and efficiencies of the MAb-catalyzed and BSA-catalyzed reactions are discussed in the context of the hydrophobic sites and intrinsic reactivity of the protein surfaces, and the induction of groups on the Ab to enhance the enzymatic function. Index Entries: Albumin; catalytic antibody; steroid; carbonate; phosphate hapten.