Cultures from patients with chronic bronchitis frequently grow hemophilus or pneumococcus. Thirty-two patients with well established chronic bronchitis were treated for 3 months at a time with 1.0 gm. daily of tetracycline, alternated with 3 months of indistinguishable placebos in a double blind study. About onethird as many exacerbations occurred during tetracycline treatment as during placebo treatment. In some patients, daily cough, sputum volume, and dyspnea were somewhat improved during tetracycline treatment. In individual patients there was a good correlation between eradication of pneumococcus and hemophilus and reduction of acute exacerbations. One patient had a fatal acute bronchitis due to an antibioticresistant staphylococcus during tetracycline therapy; otherwise, there was no evidence of replacement of normal flora during tetracycline treatment.