Human bronchial kallikrein (HBK) is the major kinin-generating enzyme in the airways of asthmatic subjects. We examined the in vitro inhibition of HBK and its in vivo regulation in asthma. HBK kininogenase activity was measured by cleavage of radiolabeled purified high molecular weight kininogen (HMWK) (125I HMWK). Functional inhibition of the enzyme was present after co-incubation with either normal human plasma (NHP) or preparations of alpha-1-proteinase inhibitor (alpha 1-PI). Stable HBK inhibitor complex was detected at 92 K by immunoblotting employing anti-HUK as a probe. Anti alpha 1-PI probes did not demonstrate the 92 K complex under identical conditions, raising the possibility of an additional co-purifying protein involved in HBK binding. alpha 1-PI antigenic and activity levels were measured in asthmatic bronchoalveolar lavage (BAL) fluid. In only a single sample of the six examined was alpha 1-PI activity detected. In two of the remaining five samples, low levels of alpha 1-PI antigen were present which were functionally inactive. In the BAL fluid containing alpha 1-PI activity, progressive functional inhibition of kininogenase activity correlating with HBK inhibitor complex formation occurred during BAL fluid incubation. In contrast, BAL fluid lacking alpha 1-PI activity failed to show progressive kininogenase inhibition or complex formation under identical conditions. In all experiments, the time course of HBK inhibitor interaction was markedly prolonged, requiring 150-min co-incubation with either NHP or the alpha 1-PI preparation for greater than 80% inhibition of function and near maximal complex formation. The slow rate of inhibition of HBK may allow for its continued activity and kinin generation in the airways of asthmatic subjects.