Human neutrophils triggered with phorbol myristate acetate or opsonized zymosan particles released a metalloproteinase (MP) capable of cleaving and inactivating alpha-1-proteinase inhibitor (alpha-1-PI). Sequence analysis of the amino acids in proteolyzed, native alpha-1-PI revealed a unique single cleavage site between Phe-352 and Leu-353. An analysis of the process regulating the enzyme's activity revealed that the neutrophil MP was released from cells in a latent form whose activation was tightly linked to the generation of hypochlorous acid. These results indicate that human neutrophils use chlorinated oxidants to activate a latent MP that is capable of proteolytically inactivating alpha-1-PI by cleaving the antiproteinase at a unique point in its inhibitory site region.