A method for determining whether a microorganism produces an AmpC β-lactamase is disclosed in which a culture of a microorganism suspected of producing a β-lactamase that inactivates a β-lactam-containing antibiotic is admixed with an effective amount of each of i) a β-lactam-containing antibiotic, ii) a β-lactamase inhibitor to which AmpC β-lactamase is resistant, and iii) a permeabilizing agent for the microorganism present in a non-growth-inhibiting microorganism-permeabilizing amount to form an assay culture. That assay culture in maintained under appropriate culture conditions and for a time period sufficient to determine the interaction of the microorganism with the AmpC β-lactamase resistant inhibitor and antibacterial compound, and thereby determine the presence of an AmpC β-lactamase, wherein a positive test indicates the presence of an AmpC β-lactamase.