Monoclonal antibodies catalyzing lysis of 4-nitrophenyl esters have been created using a phosphonate as hapten in the immunization. Among 960 hybridomas screened, 3 were found to produce antibodies catalyzing hydrolysis of 4-nitrophenyl butanoate (1). Two of the antibodies accelerate the reaction by factors of 1.3×104and 1.1×104, respectively, while the third antibody is significantly less effective. The two catalytically most effective antibodies also catalyze perhydrolysis of 1,i.e., lysis with hydrogen peroxide, to generate peroxybutanoic acid. Perhydrolysis was found to be the predominant reaction even in dilute solutions of hydrogen peroxide. Both antibodies also catalyze hydrolysis of both 4-nitrophenyl hexanoate and decanoate, but do not catalyze hydrolysis of 4-nitrophenyl acetate. The antibodies are more selective with respect to the aromatic part of the substrate as they do not catalyze hydrolysis of 2-nitrophenyl butanoate or 4-sulfophenyl nonanoate. Furthermore, neither of the antibodies catalyze hydrolysis of pre-formed peroxybutanoic acid.