The structures of two major components, leupeptins Pr-LL and Ac-LL, isolated from cultures of various species of Actinomycetes, have been investigated. Mild acid hydrolysis of leupeptins Pr-LL and Ac-LL gave propionyl-and acetyl-L-leucyl-L-leucine, whose structures were established by syntheses. Complete acid hydrolysis of leupeptin acids derived by oxidation of leupeptins gave L-leucine and DL-arginine. The structures of leupeptins Pr-LL and Ac-LL were determined to be propionyl- and acetyl-L-leucyl-L-leucyl-DL-argininal, respectively, and confirmed by the syntheses. Propionyl-and acetyl-L-leucyl-L-leucyl-L-arginine methyl esters were reduced to their alcohols with lithium borohydride and then oxidized to respective leupeptins Pr-LL and Ac-LL by sulfoxidecarbodiimide reaction.