Abstract The relation between the structure of haptens and the esterolytic activities of antibodies was investigated. We synthesized two phenylalanine analogs, the negatively charged phosphonate derivative 1 and the neutral phosphonamidate derivative 2. Seventeen out of 41 monoclonal antibodies generated against the hapten 1 hydrolyzed the relevant phenylalanine ester R- 12 . On the contrary, none of 27 monoclonal antibodies generated against the hapten 2 had catalytic activity. An ab initio study of the structural and electronic properties of the modeled haptens showed that the value of the negative electrostatic potential around the phosphonyl oxygen was an important factor affecting the induction of esterolytic antibodies.