Inhalation of purified cotton bract extract has been shown to induce bronchoconstriction and increase the number of neutrophils obtained by bronchoalveolar lavage in human subjects (Cooper et al., 1986). The objective of the present study was to determine the effect of this extract in the guinea pig animal model. An aqueous extract of cotton bract was prepared by the method of Buck and Bouhuys (1980). This extract contained low endotoxin (7.9 EU/mg) and nondeductible FMLP levels. Guinea pigs were exposed for 3 hrs to aerosols nebulized from a solution containing 30mg of cotton bract extract per ml of sterile water. Exposure dosage was controlled at one of three levels by varying the following generator parameters: nebulizer output, flow rate, and the amount of diluent air. No changes in pulmonary parameters occurred at the lower two doses of purified cotton bract extract. At the highest exposure dose, however, increases in breathing rate, macrophage superoxide release, as well as bronchoalveolar lavage yield of total cells, red blood cells, lymphocytes, and granulocytes were noted. Although, these pulmonary responses were qualitatively similar to those observed in guinea pigs exposed to crude cotton dust extract, purified cotton bract extract exhibited somewhat lesser potency.