Azithromycin (Zithromac), a 15-membered ring macrolide antibacterial agent, was approved to be manufactured in Japan in March 2000. It showed good in vitro and/or in vivo antibacterial activities against Staphylococcus aureus, Streptococcus pneumoniae, Streptococcus pyogenes, Peptostreptococcus micros, Haemophilus influenzae, Moraxella catarrhalis, Mycoplasma pneumoniae and Chlamydia pneumoniae. Its activity against H. influenzae was particularly more potent than that of currently used macrolide antibacterial agents. After oral administration to patients, azithromycin was readily absorbed and became widely distributed throughout the body, achieving higher concentrations in tissues and phagocytic cells than in serum or plasma. Its distribution into phagocytes was as high as more than 10 times that of erythromycin, and azithromycin was readily released from phagocytes in the presence of S. aureus. In experimentally infected mice, the concentration of azithromycin was higher in infected tissues than in uninfected tissues, which indicated that azithromycin was selectively delivered to infected tissues by migrating phagocytes. These pharmacological and pharmacokinetic properties were reflected in good clinical results for the treatment of respiratory infections and other infections with once daily dosing for 3 days.