An effect of a combination of ampicillin (ABPC) and dicloxacillin (MDIPC) on penicillins and cephalosporin-susceptible E. coil was determined. Minimum inhibitory concentrations of the combined drug were as same as that of ABPC, but bacterial growth was inhibited by the combined drug stronger than ABPC. Low activity of β-lactamase was detected in sonicated E. coli 0.225 cells, and this activity was inhibited by relatively low concentration of MDIPC. We considered that a weak synergism of the combined drug might attribute to the inhibition of the A-lactamase activity by MDIPC.