The acylation step of the β-lactam hydrolysis by a class A β-lactamase has been modeled by means of the quantum chemical PM3 method. A model of the catalytic site of the protein has been built using two segments of three and four amino acids, respectively, which include four of the most important residues of the enzyme. The mechanism is based on that proposed by Strynadka et al. in 1992 (Strynadka et al., Nature 359 (1992) 700–705). The results confirm that Lys-73 is expected to be unprotonated in the wild-type enzyme, and therefore acts as a general base in the acylation step of the reaction. Previous contradictory results are discussed, and a new proton transfer pathway from Lys-73 to β-lactam nitrogen is proposed.