Antibody NPN43C9 catalyses the hydrolysis of a p-nitrophenyl ester and p-nitroanilide. Kinetic investigations show that hydrolysis of the ester involves at least two steps whose relative importance changes with pH. We propose that the reaction proceeds via formation of an acyl intermediate that deacylates through attack by hydroxide. The rate-limiting step changes from product release at high pH to hydroxide ion-mediated hydrolysis at low pH. This multistep pathway shares similarities with that used by serine proteases.