Hepatocyte growth factor (HGF) activator is a serine protease responsible for proteolytic activation of HGF in response to tissue injury and thus plays an important role in the regulation of biological functions of HGF in regenerating tissue. We previously purified an inhibitor of HGF activator (HGF activator inhibitor type 1, HAI-1) from the conditioned medium of a human stomach carcinoma cell line MKN45 and cloned its cDNA. HAI-1 is a novel member of the Kunitz family of serine protease inhibitors. In the present study, we purified a second type of HGF activator inhibitor (HAI-2) from the conditioned medium of MKN45 cells and molecularly cloned its cDNA. The cDNA sequence revealed that HAI-2 is derived from a precursor protein of 252 amino acids and contains two Kunitz domains, indicating that HAI-2 is also a member of the Kunitz family of serine protease inhibitors. The primary translation product of HAI-2 has a hydrophobic sequence in the COOH-terminal region, suggesting that, like HAI-1, HAI-2 is produced in a membrane-associated form and secreted in a proteolytically truncated form. Because HAI-2 and HAI-1 are potent inhibitors specific for HGF activator, they may be involved in regulation of proteolytic activation of HGF in injured tissues.