We measured the sizes of the inhibition zones of oral beta-lactam antibiotics for Streptococcus pneumoniae, Streptococcus pyogenes and Haemophilus influenzae in the presence of beta-lactamase-producing-Moraxella (Branhamella) catarrhalis by the agar double-layer method. The sizes of the zones of amoxicillin for S. pneumoniae alone were the largest, followed in a descending order by those of cefixime and cefaclor. In the presence of 10(7) CFU/ml of M.(B.) catarrhalis, however, significant reduction of the sizes of the zones was seen with amoxicillin and cefaclor; inhibition with cefixime was nearly unchanged. Similar results were observed in those for S. pyogenes. These variable findings were attributed to the difference in stability of these drugs to the beta-lactamase produced by M.(B.) catarrhalis. When the susceptibility of H. influenzae in the presence of 10(8) CFU/ml of M.(B.) catarrhalis to cefixime, cefoteram, cefpodoxime, cefotiam and cefuroxime was examined, the sizes of the inhibition zones of all the drugs were reduced by the presence of 10(8) CFU/ml of M.(B.) catarrhalis, but those of cefixime were the largest of all the drugs tested. Our agar double-layer method is simple and useful for evaluating the influence of beta-lactamase-producing organism, as M.(B.) catarrhalis, on the disk susceptibility of other pathogens to antibiotics.