Protease-inhibitory activity of recombinant Ha-ras gene products (Ras) toward papain and cathepsins B and L was investigated. v-Ha-Ras showed more potent inhibitory activity toward cathepsin B as compared with c-Ha-Ras. We have also investigated protease-inhibitory activity of c-Ha-Ras mutants with point mutations in amino acids between positions 23 and 50. Inhibitory activity of Ras toward papain and cathepsin L was not largely altered among mutants. However, the inhibitory activity toward cathepsin B was significantly impaired by a mutation at position 43, 44, 45 or 48. These results suggest that 43Gln-Val-Val sequence plays an important role at least to inhibit cathepsin B.