Between 1 January and 31 March 1991, 20 laboratories in England and Scotland sent a total of 413 consecutive clinical isolates of Moraxella catarrhalis to The London Hospital Medical College (LHMC). After confirmation of identity, the susceptibility of all isolates to 11 antimicrobial agents was determined. Of the 375 (90.8%) isolates which were found at LHMC to be beta-lactamase-positive, 174 produced zones of inhibition around 2 micrograms ampicillin disc which were greater than or equal to 20 mm in diameter and 252 were inhibited by less than or equal to 0.5 mg/L of ampicillin. However, 71 of these 375 had been reported to be ampicillin-susceptible by peripheral centres. While beta-lactamase had not been detected in 35 of these 71 isolates, the other 36 had been reported to be ampicillin-susceptible and beta-lactamase-positive. All 38 beta-lactamase-negative isolates produced zones greater than or equal to 30 mm in diameter and were inhibited by less than or equal to 0.06 mg/L of ampicillin. No M. catarrhalis isolate was found to be resistant to co-amoxiclav, tetracycline, chloramphenicol or cefaclor. Two strains showed intermediate susceptibility to erythromycin (MIC 1 mg/L) and 27 required greater than or equal to 32 mg/L of sulphamethoxazole for inhibition. Resistance to trimethoprim was uniform (MICs 2-128 mg/L). Two isolates showed intermediate susceptibility to cefixime (MIC 2 mg/L) but none was resistant to the new oral cephalosporin cefetamet or to the oral carbacephem loracarbef.(ABSTRACT TRUNCATED AT 250 WORDS)