Clavulanic acid analogs lacking the C-3 carboxyl group are potent inhibitors of both plasmid and chromosomally mediated β-lactamases. They exhibit only low intrinsic antibacterial activity, but potentiate the activity of ampicillin and cephaloridine against β-lactamase producing Escherichia coli and Enterobacter cloacae in vitro. No synergism was observed in β-lactamase negative strains. The E. coli TEM 1 and the E. cloacae P99 enzymes are inhibited in a progressive and irreversible manner by these compounds.