Aims: To understand the structure-activity relationship of chensinin-1, a antimicrobial peptide (AMP) with an unusual structure, and to develop novel AMPs as therapeutic agents. Methods and Results: A series of chensinin-1 analogues were designed and synthesized by one to three replacement of glycines with leucines at the hydrophilic face of chensinin-1 or rearrangement of some of the residues in its sequence. Circular dichroism spectroscopy showed that the analogues adopted a-helical-type conformations in 50% trifluoroethanol/water but adopted b-strand-type conformations in 30 mmol l À1 sodium dodecyl sulphate. The anti-microbial activities of the peptides against Gram-positive bacteria increased 5-to 30-fold, and these increases paralleled the increases in the peptides' hydrophobicities. Their haemolytic activities also increased. Amphipathicities had little influence on the bactericidal activity of chensinin-1. All peptides caused leakage of calcein entrapped in negatively charged liposomes although with different efficiencies. The peptides did not induce leakage of calcein from uncharged liposomes. Conclusions: Peptide adopted an aperiodic structure can improve the antimicrobial potency by increasing peptide hydrophobicity. Its target is bacteria plasma membrane. Significance and Impact of the Study: Chensinin-1 can act as a new lead molecule for the study of AMPs with atypical structures.