The in vitro activity of ampicillin, cefotaxime, meropenem, panipenem, imipenem and biapenem was assayed using ampicillin-susceptible, beta-lactamase-positive and beta-lactamase-negative ampicillin-resistant (BLNAR) Haemophilus influenzae isolated recently in Japan. Against ampicillin-susceptible isolates, cefotaxime was the most potent (MIC(90) 0.016 mg/mL). Both cefotaxime and meropenem (MIC(90) of both, 0.5 mg/L) were the most potent against beta-lactamase-positive isolates. Against BLNAR isolates, meropenem (MIC(90) 0.5 mg/L) was the most potent. In murine bronchopneumonia caused by ampicillin-susceptible and BLNAR H. influenzae, cefotaxime showed the best efficacy, followed by meropenem. Our results indicate that meropenem could be a useful intravenous agent for infections caused by H. influenzae, including BLNAR strains.