Macrolidc antibiotics, commonly used in upper and lower respiratory tract infections, are inconsistently active against Haemophilus influenzae . The new azalide, azithromycin, was compared with erythromycin and roxithromycin against this pathogen. Azithromycin (MIC range 0.06–1 mg/l) was four to eight times more potent than erythromycin (MIC range 0.5–8 mg/l) and roxithromycin (MIC range 0.5–16 mg/l). At 1 mg/l, 100% of the strains of H.influenzae were inhibited by azithromycin compared with 16% with erythromycin and 5% with roxithromycin. Azithromycin exhibited a rapid bactericidal effect, with a 99.9% kill at 4h. The MBC was equal to or up to four-times greater than the MIC.