In a double‐blind crossover trial, the effect of a new mucolytic drug, the sodium salt of 2‐mercaptoethane sulfonic acid (Mistabron) (6 ml, 200 mg per milliliter) was compared with 0.9% sodium chloride (6 ml) over 16 consecutive days in 31 sub;ects with chronic bronchitis. Drugs were assigned randomly and were self‐administered by nebulization. Pulmonary function measurements were made pre‐nebulization, 5 minutes, and 45 to 60 minutes after nebulization each day. Symptomatic assessment of cough, sputum production, and dyspnea were recorded daily. No changes attributable to either treatment were noted in blood pressure, electrocardiographic tmcings, chest radiographs, blood chemistries, and urinalyses during the study. Pulmonary function tests (which included whole body plethysmography) showed clinically insignificant changes, though most subjects claimed to have experienced clinical improvement after taking 2‐mercaptoethane sUlfonic acid. Sputum production was measured in the last 18 patients who entered the study, and 2‐mercaptoethane sulfonic acid consistently resulted in greater production than saline. The difference was significant (p = 0.001). It is concluded that 2‐mercaptoethane sulfonic acid (20%) is as safe as an equivalent volume of normal saline administered by inhalation to subjects with chronic bronchitis, and that it is a more effective mucolytic agent.