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This study examined plasma exudation into the bronchial lumen after allergen challenge. A novel low-trauma technique was developed to challenge and lavage a medium-sized lingular or middle lobe bronchus. Eleven subjects with challenge-assessed pollen-sensitive asthma were allocated to fiberbronchoscopy in the supine position. In the control bronchus 0.5 ml diluent was instilled. The bronchus was occluded proximally 3 min later by inflation of a balloon, and lavage was carried out twice with 25 ml saline. Incremental doses of allergen solution (0.5 ml) were then instilled in the contralateral lung. The challenge continued until a clearly visible bronchial reaction occurred and was immediately followed by the same lavage as on the control side. The lavage liquids were analyzed for the presence of plasma exudation and mast cell activation indices. On the allergen-challenged side, tryptase, reflecting mast cell activation, was increased by 150% (p0.01) compared with the control side. Fibrinogen (mol wt 340,000), reflecting large protein exudation, was increased by 840% (p0.05), and N-alpha-tosyl-L-arginine-methyl esterase activity, reflecting both large protein exudation and mast cell activation, increased by 480% (p0.01). The level of albumin (mol wt 69,000), the major luminal protein under baseline conditions, increased but not significantly. We conclude that activation of mast cells and luminal entry of little sieved plasma exudates occur early after endobronchial allergen provocation in human subjects with allergic asthma.