The plasma protein alpha 2-antiplasmin is the main physiological inhibitor of the serine protease plasmin, which is responsible for the dissolution of fibrin clots. We have determined the primary structure of mature human alpha 2-antiplasmin by DNA sequencing of overlapping cDNA fragments prepared from human liver mRNA. cDNA clones were identified by hybridization with a 48-base pair deoxyoligonucleotide probe deduced from the sequence of a 16-amino acid peptide of alpha 2-antiplasmin. Mature human alpha 2-antiplasmin contains 452 amino acids. It is homologous (23-28%) with five other proteins belonging to the serine protease inhibitor (serpin) superfamily. Its reactive site, i.e. the peptide bond cleaved by reaction with its primary target enzyme, plasmin, consists of Arg364-Met365. This dipeptide corresponds to the reactive site Met358-Ser359 of the archetypal serpin, alpha 1-antitrypsin.