It was previously found that a cationic amphiphilic peptide, Ac-(Leu-Ala-Arg-Leu)3-NHCH3 (43), caused the destabilization of a phospholipid membrane and showed strong antibacterial activity [Lee et al. Biochim. Biophys. Acta 1986; 862: 211–219]. In order to investigate the effect of changing α-helix propensity, hydrophobicity and basicity in 43 on the peptide conformation and activity, the 43 analogs, [Gly (or Val)6]43, [Gly (or Val)2,6]43, [Gly (or Val)2,6,10]43, [Gln3]43, [Gln3,7]43 and [Gln3,7,11]43 were synthesized. Except for [Val2,6]43 and [Val2,6,10]43, which mainly formed a β-structure, other peptides formed an α-helix and showed moderate membrane-perturbing activity toward neutral and acidic lipid vesicles. All the peptides other than [Val2,6,10]43 and [Gln3,7,10]43 had the antibacterial activity comparable with that of 43. The relationship between the membrane-perturbing activity and the antibacterial activity was not always parallel. Conclusively, the Ala→Val substitution in 43 causes the change of peptide conformation and the presence of a cationic amino acid residue is necessary for the antibacterial activity. Copyright © 1999 European Peptide Society and John Wiley & Sons, Ltd.