SummaryS-carboxymethylcysteine was compared with bromhexine in a double-blind between-patient clinical trial in bronchiticpatients with mucoid sputum. Oral S-carboxymethylcysteine, administered as a 5% w/v syrup was given in a dosage of 750 mg. three times daily for 10 days and compared with oral bromhexine syrup (4 mg./5ml.) in a dosage of 16 mg. three times daily. Treatment with both drugs resulted in a significant increase in sputum volumes and a significant increase in thelpourability' of sputum. An improvement was also noted with both drugs in the subjective measures of ease of expectoration, severity of coughing, and sputum consistency. However, there was no improvement in ventilatory capacity or peak expiratory flow rates. S-carboxymethylcysteine was preferred significantly more by the patients' and the clinician's assessment. Bromhexine was not considered to improve the overall respiratory state as assessed by clinician or patients themselves. Whereas both drugs demonstrated valuable therapeutic responses, and were equally effective in their action on improving expectoration, sputum volumes and sputum pourability, S-carboxymethylcysteine produced a beneficial response in complementary parameters at least 4 days before a notable response to bromhexine treatment. Side-effects only occurred in 2 patients. One patient was withdrawn because of severe nausea and vomiting whilst receiving bromhexine and another patient experienced transient abdominal discomfort whilst receiving S-carboxymethylcysteine.