Brobactam is a synthetic inhibitor of beta-lactamases produced by both gram-positive and gram-negative bacteria. Brobactam potentiates the antibacterial activity of ampicillin against a wide range of clinically important bacterial strains which produce beta-lactamase. No resistant sub-population was observed amongst the strain s of staphylococci studied, and the development of resistance in vitro was not recorded in individual strains of Staphylococcus aureus and Escherichia coli exposed to subinhibitory concentrations of ampicillin/brobactam. Reduced sensitivity was observed in the case of one strain of M. morganii, which was known to produce an inducible chromosomal cephalosporinase.