The expression of beta-lactamases is the most common form of bacterial resistance to beta-lactam antibiotics. To combat these enzymes, agents that inhibit (e.g. clavulanic acid) or evade (e.g. aztreonam) beta-lactamases have been developed. Both the beta-lactamase inhibitors and the beta-lactamase-resistant antibiotics are themselves beta-lactams, and bacteria have responded to these compounds by expressing variant enzymes resistant to inhibition (e.g. IRT-3) or that inactivate the beta-lactamase-resistant antibiotic (e.g. TEM-10). Moreover, these compounds have increased the frequency of bacteria with intrinsically resistant beta-lactamases (e.g. AmpC). In an effort to identify non-beta-lactam-based beta-lactamase inhibitors, we used the crystallographic structure of the m-aminophenylboronic acid-Escherichia coli AmpC beta-lactamase complex to suggest modifications that might enhance the affinity of boronic acid-based inhibitors for class C beta-lactamases. Several types of compounds were modeled into the AmpC binding site, and a total of 37 boronic acids were ultimately tested for beta-lactamase inhibition. The most potent of these compounds, benzo[b]thiophene-2-boronic acid (36), has an affinity for E. coli AmpC of 27 nM. The wide range of functionality represented by these compounds allows for the steric and chemical "mapping" of the AmpC active site in the region of the catalytic Ser64 residue, which may be useful in subsequent inhibitor discovery efforts. Also, the new boronic acid-based inhibitors were found to potentiate the activity of beta-lactam antibiotics, such as amoxicillin and ceftazidime, against bacteria expressing class C beta-lactamases. This suggests that boronic acid-based compounds may serve as leads for the development of therapeutic agents for the treatment of beta-lactam-resistant infections.