Elafin, an elastase-specific inhibitor isolated from human skin, and its related peptides were synthesized by the solution procedure, and their inhibitory activities were measured against various enzymes. During the oxidative folding reactions of the reduced peptides, the ratio of the active product to the inactive product was varied by changing the concentration of guanidine HCl and the amount of redox reagents. The disulfide structures of fully active synthetic elafin and the inactive product were determined by amino acid analysis, gas-phase sequencing and mass spectrometry of their proteolytic fragments. The relationship between structure and inhibitory activities and/or the folding reaction was examined and the amino terminal part of the elafin molecule was found to have a great influence on the folding reactions, but not on the inhibitory activities.