Specific enzyme inhibitors of microbi&l origin have been studied in the authors' laboratory and as reported in previous papers, leupeptins^5) inhibiting plasmiri, trypsin, kallikrein and papain, pepstatin6~f° inhibiting acid proteases, and chymostatin1^ inhibiting chymotrypsins have been isolated. These inhibitors were found in culture filtrates of various species of actinomycetes* In this paper, the isolation and properties of antipain are reported.