SummaryTwenty-six patients with acute exacerbations of chronic bronchitis requiring admission to hospital were treated with clindamycin 300 mg. or 450 mg. 6-hourly for 10 days to 3 weeks. Four were withdrawn because of side-effects. In 8 of the 22 completing the study, pneumococci were present in the pre-treatment sputum. With treatment, the organism was eradicated within 24 hours and in 5, the sputum became mucoid. All improved clinically although Haemophilus influenzae organisms were isolated at some stage from the sputum of all but one of these patients. Only 1 of the 14 whose sputum yielded predominantly haemophilus without pneumococci achieved sputum conversion, only 5 improved clinically and 3 deteriorated considerably in spite of intensive physiotherapy. Bacteriological eradication of Haemophilus influenzae did not occur when in vitro testing suggested sensitivity.It is concluded that clindamycin fails to eradicate Haemophilus influenzae from the sputum of patients with acute exacerbation of chronic bronchitis, irrespective of in vitro sensitivity. Its effectiveness in patients with pneumococcal and mixed pneumococcal infections suggests that the pneumococcus may be of primary pathogenic significance in such patients.