Azithromycin is a molecule of the macrolide family, belonging to the azalides class. Several of its characteristics allow for its use in the treatment of the community-acquired lower respiratory tract infections. Commonly isolated pathogens in bronchial infections are most frequently H. influenzae, S. pneumoniae, M. catarrhalis, C. pneumoniae and M. pneumoniae, and more rarely or in the context of a particular background, S. aureus, Gram negative bacteria and L. pneumophila. MIC90 of these germs is generally low or slightly elevated, displaying an inhibitory activity of the azithromycin on these bacteria. Nevertheless, the frequency of macrolide-resistant S. pneumoniae is not negligible and this germ must be considered as inconstantly susceptible to the macrolide family. Pharmacokinetics studies evidenced from high to very high azithromycin concentrations in the pulmonary tissues, reaching values well above MIC of pathogens commonly isolated. Given the long half-life, these concentrations persist a long time after oral administration. As azithromycin concentrates much in polymorphonuclear leucocytes, they release azithromycin after having migrated into the infectious site by chimiotactism, thus allowing to increase the antibiotic concentration at infection site. These requirements have been confirmed in vivo in animal models and in clinical studies. Two experimental models on macrolide susceptible S. pneumoniae, and H. influenzae evidenced a better activity of azithromycin in comparison to other macrolides tested against these two germs.