The clearance of (99TM)-Tc-DTPA from the airways was employed to assess the permeability of the respiratory tract epithelium. Exposure of guinea-pigs to a single dose of nebulised extract of cotton dust which induced bronchoconstriction did not affect the to.5 for (99TM)--Tc-DTPA removal. However, following exposure of the animals to daily (Monday through Friday) doses of cotton or flax dust extracts for 4 weeks, significantly enhanced clearance of the labelled marker occurred. This suggests that chronic exposure of these animals to the dust extracts increase airway permeability.