Leupeptin, a tripeptide isolated from Streptomyces sp. with a structure of Ac-L-Leu-L-Leu-L-Argal, was reported as a strong inhibitor of trypsin-family proteases in 1969. Leupeptin is active against various proteases, including papain, trypsin, plasmin, kallikrein (IC50 0.086, 0.22, 1.0 and 1.0 x 10-5M, respectively), as well as serine potease, calpain and cathepsin B. Leupeptin strongly inhibits the differentiation of the opportunitstic mycosis, Scedosporium apiospermum.