We measured the sizes of the inhibition zones of oral β-lactam antibiotics for Streptococcus pneumoniae, Streptococcus pyogenes and Haemophilus influenzae in the presence of β-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 107 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 β-lactamase produced by M. (B.) catarrhalis. When the susceptibility of H. influenzae in the presence of 108 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 108 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 β-lactamase-producing organism, as M. (B.) catarrhalis, on the disk susceptibility of other pathogens to antibiotics.