Leupeptins which have anti-plasmin activity, anti-inflammatory effect and inhibit trypsin, papain and blood coagulation have been obtained from cultured broths of various species of Actinomycetes (including Streptomyces) and named leupeptins. Two leupeptins, Pr (C₂₁H₄₀O₄N₆) and Ac (C₂₀H₃₈O₄N₆), were isolated and characterized. They gave their di-n-butyl acetals, dihydroleupeptins and leupeptin acids. Physical and chemical properties of these leupeptins and their derivatives were described. In the present paper, production, isolation and characterization of leupeptins produced by Streptomyces roseus MA839-A1 are described.