Background: Microvascular-epithelial exudation of bulk plasma may characterize inflammatory airway diseases. This study compares the acute allergen challenge–induced mast cell and exudative responses in nasal and bronchial airways. The focus is on ⍺₂-macroglobulin as an index of luminal entry of plasma exudates. Methods: Separate nasal and bronchial allergen challenges were carried out outside the pollen season in eight patients with pollen-induced seasonal allergic rhinitis. The levels of different-sized plasma proteins (albumin molecular weight, 66,000 d and ⍺₂-macroglobulin molecular weight, 725,000 d) and tryptase were determined in pre- and postchallenge nasal lavage and bronchoalveolar lavage (BAL) fluids. Diluent and increasing doses of allergen were sprayed into the right nasal cavity, and each challenge was followed by a nasal lavage (volume, 15 ml) with a “nasal pool” device (recovery, >80%). Endobronchial allergen challenge (individual doses) and BAL (volume, 2 × 25 ml) were performed in a lobe bronchus through a fiberoptic bronchoscope (recovery, 30%). Saline challenge and BAL were carried out in the contralateral lung as control. Results: The levels of albumin, ⍺₂-macroglobulin, and tryptase increased dose-dependently in postchallenge nasal lavage fluids (p < 0.05) and correlated to nasal symptoms. In particular, albumin and ⍺₂-macroglobulin correlated (r = 0.98, p < 0.001). Both ⍺₂-macroglobulin and tryptase, but not albumin, were increased in BAL fluids from the allergen-challenged side (p < 0.05). Conclusion: Local allergen challenge causes luminal entry of tryptase and ⍺₂-macroglobulin in the nose and bronchi of patients with allergy. We suggest that mast cell and plasma exudation responses may be similar in human nasal and bronchial airways and that albumin levels (in BAL fluids) may not well reflect the exudation process in bronchial airways. (J ALLERGY CLIN IMMUNOL 1995;96:239-46.)