The distribution of β-lactamases in a group of 20 epidemiologically well defined Acinetobacter baumannii clinical isolates and the in vitro activity of Syn 2190, a novel β-lactamase AmpC inhibitor, were determined. Twenty-five per cent of the strains carried and expressed a TEM-type β-lactamase, whereas 35% had an OXA-type β-lactamase. In nine out of 11 (82%) ceftazidime-resistant and four out of 13 (30.7%) cefepime-resistant strains, the MIC of these β-lactam antibiotics decreased when determined in the presence of Syn 2190. Thus, our results suggest that in a high percentage of A. baumannii clinical isolates the increased production of AmpC, in combination or not with other resistance mechanisms, contributes to the resistance pattern in A. baumannii to β-lactams.