Inhalation of aqueous extracts of cotton bract by human volunteers causes acute airway constriction. Constriction is observed by decreases in the lung function values FEV1.0 and MEF40%(P). The well-known airway constrictors, histamine and endotoxin cannot account for the constricting activity of bract extracts. Our purified extracts do not contain either of these components and yet retain 64% of the original constricting activity of the crude bract extract. Activity also remains after lacinilene, tannin, and terpenoid removal. The active component in our purified bract extract has a molecular weight of 1,000 daltons or less. This extract shows UV absorbance at 225nm and a broad triple peaked absorbance in the 262nm region, and it fluoresces at 430nm. An HPLC profile with detection at 280nm showed that with the exception of 2 minor peaks, all fractions of our extract eluted with water alone. Further chromatography by HPLC shows 11 distinct peaks within this fraction. Primary amine analysis by HPLC shows that primary amines appear not to be involved with constrictor activity, but a small peptide remains a likely possibility. Another possibility is a leukotriene-like compound.