Mycobacterium avium infection is a frequent complication during the late stage of AIDS. M. avium is resistant or poorly susceptible to classical antituberculosis drugs. Some new macrolide antibiotics such as clarithromycin or azithromycin are bactericidal against M. avium, and their use has dramatically improved the prognosis of this infection. In vitro, azithromycin has MICs against M. avium ranging from 4 to 64 MIC50 being 16 mg/l and a MIC90 being 32 mg/l. Despite low concentrations in serum, close to 0.4 mg/l, very high intracellular concentrations of azithromycin, above the MIC, may be achieved. Azithromycin is active in vitro in the model of macrophage infection. Furthermore, azithromycin is active against murine experimental infection with M. avium. Clinical studies conducted among patients with AIDS have shown that azithromycin was active against disseminated M. avium infection, with a dose of 600 mg/d. Ongoing studies are designed to better determine the ideal dose, to compare its activity with that of clarithromycin and to determine the antibiotics that could be combined to prevent the selection of resistant mutants. Other ongoing studies are evaluating the efficacy of azithromycin for the chemoprophylaxis of M. avium infection in HIV infected patients with a CD4T lymphocyte concentration lower than 100/mm3.