The in vitro inhibitory activity of a combination of amoxicillin and gentamicin was assayed by the microdilution technique in liquid medium on 8 amoxicillin-resistant strains, with the addition of clavulanic acid at a concentration of 4 micrograms/ml. A synergistic effect was demonstrated when clavulanic acid was added. The FIC index was particularly low for the 2 Staphylococcus aureus strains tested (0.19 and 0.25), and slightly higher for the 2 Escherichia coli (0.25 and 0.5) and 2 Proteus vulgaris (0.5 and 0.5) strains. Synergism was less apparent for the 2 Klebsiella pneumoniae strains tested (0.63 and 0.75). Without clavulanic acid, a synergistic effect was found on only three strains: one Staphylococcus aureus, one Klebsiella and one Proteus. However, the high levels of MICs at which these effects were observed preclude the clinical usefulness of the association. We conclude that the synergistic effect of the amoxicillin-gentamicin combination on susceptible strains is confirmed on amoxicillin-resistant strains when clavulanic acid is added.