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Ceftaroline is a novel oxyimino-cephalosporin, strongly active against methicillin-resistant staphylococci and pneumococci. It is active against Enterobacteriaceae too, but is labile to common β-lactamases, including AmpC and extended-spectrum types. To counteract these enzymes, ceftaroline is also being developed combined with NXL104, a β-lactamase inhibitor.Chequerboard MIC titrations were performed to determine the NXL104 concentrations needed to protect ceftaroline against β-lactamase-producing Enterobacteriaceae, most of them with ceftaroline MICs >16 mg/L.All of 60 extended-spectrum β-lactamase (ESBL) producers were susceptible to ceftaroline + NXL104, 1 + 1 mg/L, as were 5/5 Klebsiella oxytoca with high-level K1 enzyme. Among 30 Enterobacteriaceae with high-level chromosomal AmpC, 18 were susceptible at 1 + 1 mg/L, 28 at 1 + 4 mg/L and all at 4 + 4 mg/L; among 10 with plasmid AmpC enzymes, nine were susceptible at 1 + 1 mg/L and all at 1 + 4 mg/L. None of 10 isolates with combinations of AmpC or ESBL and impermeability was susceptible at 1 + 1 mg/L, but nine were susceptible at 1 + 4 mg/L and all at 4 + 4 mg/L. Among 12 with KPC carbapenemases, only two were susceptible at 1 + 1 mg/L, compared with 10 at 1 + 4 mg/L and 11 at 4 + 4 mg/L; 8/8 with OXA-48 carbapenemase were susceptible at 1 + 1 mg/L whilst 0/5 with metallo-β-lactamases were inhibited by ceftaroline + NXL104, even at 8 + 4 mg/L. NXL104 potentiated the activity of ceftaroline against many ESBL- and AmpC-negative isolates for which ceftaroline MICs were 1–4 mg/L but not those for which MICs were ≤0.5 mg/L, probably reflecting the slight lability of this cephalosporin to classical penicillinases, which were present in the former group but not the latter.At concentrations of ≤4 mg/L, NXL104 protected ceftaroline against all relevant β-lactamases except metalloenzymes.