Abstract Analogs of gramicidin S, [1,1′-bis(α-aminoisobutyric acid)]gramicidin S (18) and [1-α-aminoisobutyric acid] semigramicidin S (11), have been synthesized by a conventional solution method. The cyclization reactions of a linear pentapeptide active ester and a linear decapeptide active ester yielded exclusively a protected cyclic pentapeptide and a protected cyclic decapeptide, respectively. After deprotection of these peptides, we obtained the final products (11 and 18). The mobility of 18 in paper electrophoresis was the same as that of gramicidin S, however, the CD spectrum of 18 in a methanol solution showed a markedly different pattern from that of gramicidin S. Both analogs (11 and 18) are inactive against the Gram-positive microorganisms tested. The results suggest that the positions of the valine residues in gramicidin S require a hydrophobic amino acid having l-configuration to show activity.