A thrombin-like proteinase (THROLP) was detected colorimetrically as the main proteolytic activity (PROA) in bronchoalveolar lavage fluid (BALF) 4 and 24 h after ovalbumin aerosol (OVA) challenge of actively immunized rats. Other proinflammatory parameters increased also significantly in BALF, like chemiluminescence of leukocytes (1 h), protein and cell number (24 h after challenge). In parallel, increased bronchoconstrictions against 5-HT aerosols are detected 24 h post OVA challenge. THROLP is an indicator for plasma leakage, activation of the clotting reaction and the protracted inflammation in the airways, which induced bronchial hyperreactivity.