The primary objective of the study is to determine the percentage of patients with "therapeutic success" at the end of the cycle of antibiotic therapy (10 days), in the two treatment groups (levofloxacin and prulifloxacin). The effect of study treatments will be evaluated on the basis of a score determined in relation to the signs-symptoms of acute exacerbation of COPD (sputum purulence, sputum volume, cough, dyspnea, fever)