The in vitro activities of HMR 3647, erythromycin A, clarithromycin, azithromycin, roxithromycin, penicillin G, ampicillin, cefuroxime, trimethoprim/sulfamethoxazole, tetracycline, ciprofloxacin, and levofloxacin were determined for 1179 Streptococcus pneumoniae, 1438 Haemophilus influenzae, and 428 Moraxella catarrhalis isolated from respiratory tract specimens by 18 medical centers across Canada during 1997-1998. On a per weight basis, HMR 3647 was the most active agent tested against S. pneumoniae with MIC90s of < or = 0.12 microgram/mL for both penicillin susceptible and penicillin intermediate isolates and 0.25 microgram/mL for penicillin-resistant isolates. HMR 3647 was also highly active against M. catarrhalis (MIC90, < or = 0.12 microgram/mL), but less active against H. influenzae (MIC90, 4 micrograms/mL).