Currently, there is no Clinical and Laboratory Standards Institute (CLSI) disk diffusion method for testing Moraxella catarrhalis susceptibility. We examined 318 clinical strains of M. catarrhalis obtained as part of the SENTRY (Asia-Pacific) Antimicrobial Surveillance Program, plus two ATCC strains. MICs were determined by the CLSI standard broth microdilution method, and zone diameters were determined on Mueller-Hinton agar incubated in 5% CO(2). All strains were examined for the presence of BRO-1 and BRO-2 beta-lactamases by using molecular techniques. Tentative zone diameter interpretive criteria were successfully developed for 19 antimicrobial agents, including nine beta-lactams, using current MIC interpretive criteria where available or wild-type cutoff values where no prior criteria were available. The proposed interpretive criteria were highly accurate, with <or=0.7% very major (falsely susceptible) and <or=1.0% major (falsely resistant) errors, respectively.