Azithromycin has been approved for treatment of uncomplicated cervicitis caused by Chlamydia trachomatis infection. Chlamydia pneumoniae (TWAR), a separate genus in the same family, infects epithelial cells of the respiratory tract and causes pneumonia. This study was designed to evaluate the effectiveness of azithromycin against TWAR-infected trachea epithelial cells in culture. Cells were isolated, grown to confluency in tissue culture vessels, infected with TWAR, then treated with azithromycin 3 or 7 days after infection. Cultures were filmed using time-lapse cinematography during infection and for one week after treatment, then fixed and stained for detection of TWAR inclusions by TEM. The appearance of inclusions at the EM level was compared in treated versus non-treated cultures. We found that cultures treated with azithromycin at either day 3 or day 7 post infection contained significantly fewer chlamydial particles within their inclusions than did untreated controls.