Four novel peptides that inhibit cathepsin B, designated as Cabin-1, -2, -3, and -4, were isolated from a thermolysin digest of human plasma. After gel filtration and cation-exchange chromatography, the peptide mixture was purified by reverse-phase HPLC to isolate Cabin-1, -2, -3, and 4, with the amino acid sequences LGPVTQE, VLQSSGLYS, VVSVLT, and LVYDAY, respectively. These peptides correspond to f(64-70) of human apolipoprotein A-I for Cabin-1, f(56-64) and f(185-190) of the human immunoglobulin G gamma chain for Cabin-2 and -3, and f(66-71) of human transferrin for Cabin-4. Synthetic Cabin-1, -2, -3, and -4 showed dose-dependent inhibition of cathepsin B. Their IC50 values were 450, 500, 20, and 5.0 micromol/l, respectively. Lineweaver-Burk plots suggested that Cabin-3 is a noncompetitive inhibitor, while Cabin-4 is a competitive inhibitor. Among the N- and C-terminal deletion peptides of Cabin-2 and -4, Cabin-2(1-8), VLQSSGLY, was found to have the most potent inhibitory activity, with an IC50 of 3.8 micromol/l.