Low molecular weight proteinase inhibitors have attracted attention not only as helpful tools for the study of enzyme structure and reaction mechanisms but also as useful drugs. One of us isolated three strains of producing serine proteinase inhibitors from seawater of Aburatubo Bay of Kanagawa in Japan. Strain B-10-31 (Alteromonas sp.), the highest producer, was found to produce two types of proteinase inhibitors: marinostatin (serine proteinase inhibitors with ~1600 Da) and monastatin (thiol proteinase inhibitor with ~20,000 Da). Recently, we reinvestigated the extracellular products of this strain to discover another type of thiol proteinase inhibitors with molecular weights much lower than monastatin. In this paper, we present the purification and characterization of these smaller thiol proteinase inhibitors from marine Alteromonas sp. B-10-31. The strain was cultured, and the culture fluid was centrifuged. The supernatant was treated with Diaion HP-20 resin to adsorb the inhibitors, which were eluted with 70% methanol. The eluate was evaporated and chromatographed on a CM-Cellulofine C-500 column and an Ultra Pack WP-C18 column. Three inhibitors (A, B, C) were purified. Amino acid analysis showed no amino acids other than valine, isoleucine, and leucine in the native inhibitors; after oxidation with potassium permanganate, arginine was detected, suggesting argininal at the C-terminal. MS/MS analysis identified their structures as acetyl-leucyl-valyl-argininal (A), acetyl-leucyl-isoleucyl-argininal (B), and acetyl-leucyl-leucyl-argininal (C), which are identical to leupeptins isolated from Streptomyces. This is the first case that amino-aldehyde-containing inhibitors were isolated from culture broths other than Actinomycetes.