One hundred and thirty-two patients with purulent exacerbations of chronic bronchitis were randomly allotted to treatment in three groups. They received (a) amoxycillin 250 mg and pivmecillinam 200 mg; or (b) amoxycillin 500 mg; or (c) amoxycillin 500 mg and pivmecillinam 400 mg: three times daily for 10 days. By the 7th day of treatment there was significant improvement over amoxycillin alone for both groups given combined chemotherapy in conversion of sputum to mucoid and in general improvement; at the end of treatment results in patients given the higher doses of both antibiotics were still superior to amoxycillin alone. Patients were observed 2 to 4 weeks later, when those given amoxycillin alone relapsed much more frequently. The three treatments were well tolerated and succeeded equally in clearing potential pathogens from the sputum. Combined treatment may be superior due to synergy against Haemophilus influenzae or to the elimination of beta-lactamase producing organisms and should be investigated further.