We performed a double-blind trial of a new formulation of erythromycin base as enteric-coated pellets (Ery-Max) in the treatment of acute bronchitis in 220 patients. Subjects were randomized to receive either 500 mg twice or 250 mg four times a day for 10 days. The response to treatment as assessed clinically and by changes in sputum purulence was the same in the two groups. Thus 97% and 98% of the patients had a good or improved clinical effect. Treatment was well tolerated, but side-effects were encountered more often in the q.i.d. group (p greater than 0.05). In conclusion, the study showed that the enteric-coated erythromycin as pellets, given twice or four times daily for 10 days, was well tolerated and equally effective in acute bronchitis. Hence, the more convenient b.i.d. regimen can be recommended.