Strains of bacteria resistant to beta-lactam antibiotics have been increasing in number and are becoming troublesome in clinical medicine. The in vitro antibacterial activity of amoxicillin combined with clavulanic acid was determined on selected ampicillin-resistant clinical isolates. Synergistic effects were produced by amoxicillin with clavulanic acid against ampicillin-resistant strains of Staphylococcus aureus, Escherichia coli, Klebsiella pneumoniae, Proteus mirabilis, Proteus vulgaris, and Bacteroides fragilis. Inhibition of the beta-lactamases produced by the ampicillin-resistant strains was confirmed, especially against the penicillinases mediated by the R factor and the cephalosporinases produced by P.vulgaris and B. fragilis. The inhibitory effect of clavulanic acid against beta-lactamases was irreversible because of the high affinity of clavulanic acid to the enzymes.