A catalytic IgG (Ab2) displaying a beta-lactamase-like activity was previously obtained by using the antiidiotypic pathway: the particularity of this antibody is that it is a true antiidiotype of the beta-lactamase active site. We have previously demonstrated that this IgG has retained some of the structural information displayed by the beta lactamase active site, evident from data that polyclonal anti-Ab2 antibodies (Ab3) recognize beta-lactamase. In this article, we investigated the catalytic mechanism of the abzyme compared to that of the enzyme. The experimental data allow us to hypothesize the catalytic residues required for catalysis.