An antibacterial composition for medical use comprising 6-[D(-)- alpha -(4-ethyl-2,3-dioxo-1-piperazinylcarbonylamino)phenylacetamido]penicil lanic acid or a pharmaceutically acceptable salt thereof and a beta -lactamase-inhibiting penicillin or cephalosporin. The composition exhibits synergistic effect which is much greater than the sum of antibacterial effects of each component used alone.
