Series of eight new monocyclic analogues of trypsin inhibitor SFTI-1 was synthesized by the solid phase method. In these analogues disulfide bridge Cys3 - Cys11 present in native inhibitor was replaced by different-sized carbonyl bridges formed by the amino groups of the side chain of Lys, Orn, Dab or Dap located in positions 3 and/or 11. All analogues appeared to be potent trypsin inhibitors. The values of association equilibrium constants determined with bovine β-trypsin ranging 10(8) - 10(9) M(-1) with the highest (3.90 × 10(9) M(-1)) determined for analogue containing Lys and Dap in aforementioned positions. The obtained results clearly shown that this redox stable modification is well tolerated in the structure of proteinase inhibitor. It is worth stressing that the procedure of the introduction of carbonyl bridge into the peptide structure is straightforward and therefore beneficial for the design of new enzyme inhibitors.