Aims To understand the structureactivity relationship of chensinin-1, a anti-microbial 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 beta-strand-type conformations in 30 mmol l-1 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 anti-microbial 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.