Clavulanic acid is known to potentiate the activity of amoxycillin and ticarcillin against beta-lactamase-producing Bacteroides species. In order to assess the usefulness of the clavulanate antibiotics in mixed infections with aerobes and anaerobes, artificially associated cultures of clinically significant Bacteroides species and facultative anaerobes (Escherichia coli, Klebsiella pneumoniae, Haemophilus influenzae, Enterococcus faecalis, Staphylococcus spp. and Streptococcus spp.) were tested. The minimal bactericidal concentrations (MBC) of amoxycillin and ticarcillin, singly and in combination with clavulanate, were determined for the individual species in the associated culture and for the associated culture as a whole. In the absence of clavulanate, association experiments showed that the MBCs of susceptible strains increased in the presence of beta-lactamase-producing species, whereas the addition of clavulanate to the test systems led to MBCs which were mostly comparable to values as determined in single strain testing.