The dipeptidyl phosphonamidate analogues (4)-(12) inhibited a range of serine B-lactamases, being most efficacious against the Enterobacter cloacae P99 isolated enzyme. Synergy experiments demonstrated that the antibacterial activity of amoxycillin is potentiated against bacteria producing these enzymes, the effect again being most pronounced against the P99-producing E.cloacae N-l strain. The analogues (11) and (12) (β-amino acid C-terminal) were the most active inhibitors and synergists.