In order to assess the potential role of protease inhibitors in airways regulation in asthma, we have studied the effects of para-aminomethylbenzoic acid (PAMBA) in 74 patients with various forms of bronchial asthma. The effects were assessed in challenge tests by changes in PEF or FEV1 following oral administration of analgesics, exercise or allergen inhalation. PAMBA was used in a single blind manner with lactose as placebo. PAMBA administered orally (1.0 g) was found to reduce significantly the maximal change in FEV1, PEF following challenge in patients with analgesic-induced asthma (n = 30; p less than or equal to 0.01), exercise-induced asthma (n = 37; p less than or equal to 0.01) or allergen-induced asthma (n = 7; p less than or equal to 0.05). We suggest that PAMBA reduces the accumulation of bronchoconstrictory metabolites of arachidonic acid via inhibition of phospholipase A2.