Two acid-stable proteinase inhibitors were detected in bronchial mucus, one having a mol. weight of 30,000 (BST-30) and inhibiting trypsin, chymotrypsin and, very weakly, polymorphonuclear leukocyte elastase and the other having a mol. weight of 14,000 (BSI-14) and inhibiting leukocyte elastase and cathepsin G very strongly in addition to trypsin and chymotrypsin.BSI-30 showed an immunol. cross-reaction with inter-α-trypsin inhibitor, whereas BSI-14 did not.BSI-14 and BSI-30 showed an activity ratio of 7:3 in mucus.BSI-30 is a derivative of the humoral inter-α-trypsin inhibitor, whereas BSI-14 appears to be identical to a trypsin-elastase inhibitor from seminal plasma.