Some pseudo-peptide analogs of thiol proteinase inhibitors were synthesized by a conventional solution method. Among them, Suc-Ala-Val-Val-Ala-Ψ-(CH₂-NH)-Ala-pNA (peptide 1) and Suc-Ala-Val-Val-Ψ-(CH₂-NH)-Ala-Ala-pNA (peptide 2) showed a stronger inhibitory activity compared with parent peptide such as Suc-Ala-Val-Val-Ala-Ala-pNA. In particular, peptide 2 was about 10-fold as active as the parent peptide (IC₅₀=8 μM). Inserting Ψ-(CH₂-NH) possibly makes the inhibitor less susceptible to papain and, as a result, produces more potent inhibition.