Pefloxacin is an antibacterial agent belonging to the group of quinolones having a methyl-carboxylic acid which possesses a high activity as compared to other quinolonic drugs. In this study we tested the efficacy and tolerability of the drug in the treatment of 25 patients with environmental L.R.T.I. (Low Respiratory Tract Infections). The microbiological and clinical characteristics of the drug were tested in this study. Excellent results were obtained in 96% of the patients treated and the tolerability was also very good.