In order to investigate the proper peptide backbone conformation that is biologically active, the chemotactic peptides formyl-Met-Leu-Phe-OMe and formyl-Met-Acc6-Phe-OMe (Acc6 is the a-a disubstituted amino acid l-aminocyclohexane-1-carboxylic acid) were studied by the theoretical method PEPSEA. This study shows that the parent peptide formyl-Met-Leu-Phe-OMe has a flexible structure, and that the other conformationally constrained peptide has a tendency to form the b turn structure. It also gives evidence against the hypothesis proposing the importance of a formyl group in the interaction with the receptor.