Concentrations of amoxycillin/clavulanic acid achievable in the respiratory tract following oral dosage were assessed for in-vitro activity against beta-lactamase-producing strains of Branhamella catarrhalis and Haemophilus influenzae. In agar-dilution studies, 8 mg amoxycillin/l was required to inhibit 45 strains of beta-lactamase-producing B. catarrhalis, whereas all the strains were inhibited by 0.5 mg amoxycillin/l in the presence of 0.01 mg clavulanic acid/l. Similarly, 0.1 mg amoxycillin plus 0.05 mg clavulanic acid/l were bactericidal against beta-lactamase-producing strain of B. catarrhalis and prevented regrowth within 24 h. In tests against 43 beta-lactamase-producing strains of H. influenzae, concentrations of up to 128 mg amoxycillin/l were required for inhibition, whereas 32 strains (75%) were fully sensitive to amoxycillin (MIC 0.5 mg/l) in the presence of 0.12 mg clavulanic acid/l. These concentrations of amoxycillin/clavulanic acid were also bactericidal for a beta-lactamase-producing strain of H. influenzae. The study therefore showed that amoxycillin/clavulanic acid, at concentrations similar to those likely to be achieved in the respiratory tract following oral dosage, was bactericidal in vitro for beta-lactamase-producing isolates of B.catarrhalis and H. influenzae.