A double-blind placebo controlled study of sodium cromoglycate , beclomethasone dipropionate and salbutamol (18559949) by inhalation was designed to test their effect on respiratory symptoms and changes in ventilatory capacity in cotton mill workers in Lancashire, England. Pretrial assessment of workers was made to determine presence of byssinosis, bronchitis, dyspnea, chronic cough, productive loose cough and smoking history. Pulmonary function tests were performed on all workers and a modified prick test for the allergens, grass pollen, Dermatophagoides pteronyssinus, cat fur, Aspergillus fumigatus and extracts of cotton from a variety of sources. Pulmonary function was determined before each trial and at three and six hours after starting the work shift. Workers were asked to assess whether there were differences in chest tightness during each test. All three drugs showed some beneficial effect. Salbutamol was the most effective drug studied. Beclomethasone dipropionate was also significantly effective, particularly in workers with byssinosis or exertional dyspnea. Sodium cromoglycate was the least effective. However, workers were unable to distinguish between active and placebo drugs. Drugs, as yet, do not provide a satisfactory method of preventing ventilatory response to cotton dust. Dust control remains the principal method of reducing byssinosis.