Skov R, Frimodt-Møller N, Espersen F. In vitro antimicrobial susceptibility of Staphylococcus aureus to amoxycillin-clavulanic acid, penicillin-clavulanic acid, dicloxacillin and cefuroxime. APMIS 2002;110:559–64. Sixty-three Staphylococcus aureus isolates with a wide distribution in quantitative β-lactamase production were tested in vitro against amoxycillin and penicillin in combination with clavulanic acid to establish the influence of total amount of β-lactamase present on the ability of clavulanic acid to protect against β-lactamase degradation. The β-lactamase stability of cefuroxime and dicloxacillin was also evaluated. MIC was determined by agar dilution using Mueller-Hinton agar with both a conventional as well as a 100 times higher inoculum. The strains were tested both with and without induction of the β-lactamase production. Clavulanic acid was highly effective in protecting against β-lactamase degradation of both penicillin and amoxycillin. Even when using a high inoculum of strains with induced β-lactamase production, all strains had MICs below the NCCLS breakpoint of 4/2 mg/l for amoxycillin-clavulanic acid. Both cefuroxime and dicloxacillin were highly stable against staphylococcal β-lactamase degradation. This study encourages further in vivo evaluation of amoxycillin-clavulanic acid for severe staphylococcal infections.