In vitro activity of 4 commonly used and 5 new antibiotics was examined against 177 strains of Haemophilus influenzae. All strains were collected from various sites in patients with clinical infections. The study confirms that several newer antibiotics are useful alternatives to older drugs, as measured by in vitro activity. Ciprofloxacin and ofloxacin were the most active agents, (MIC90 0.012 micrograms/ml and 0.05 micrograms/ml respectively), followed by aztreonam (MIC90 0.1 micrograms/ml) and cefuroxime (MIC90 0.8 micrograms/ml). A new macrolide, azithromycin (CP 62,993), was more active than erythromycin, MIC90 1.6 micrograms/ml vs 6.4 micrograms/ml. Beta-lactamase production was detected in 4.5% (8/177) of the strains. In vitro activity was the same against strains collected in 1985 and 1988. No increase in beta-lactamase production was recorded.