Bactenecin 7 (Bac7), a cationic polypeptide from large granules of bovine neutrophils, exhibits antimicrobial activity mainly against Gram-negative bacteria. This peptide is characterized by high contents of Arg and Pro and by a unique sequence with an Arg-clustered region and three tandem repeats. In order to investigate the structure-function relationship of Bac7, two peptide fragments, which correspond to residues 1-17 (RRIRPRPPRLPRPRPRP, an Arg-clustered region) and 46-59 (LPFPRPGPRPIPRP, one of three tandem repeats), respectively, were synthesized. Circular dichroism (CD) measurements of the two fragments indicated that they took particular conformations, although these were not defined. These peptides can bind to acidic phospholipid bilayers without marked conformational changes. When acidic phospholipid bilayers entrapping 5,6-carboxyfluorescein were treated with the peptides, no trace of the dye leaked out, indicating that the peptides lack the ability to disrupt lipid membranes. Fragment 1-17 showed weak antimicrobial activity against several bacteria, but fragment 46-59 was almost inactive. The present results suggest that longer fragments or the entire molecule of Bac7 may be required for full expression of antimicrobial activity and that Bac7 may manifest its activity by a bacteriostatic rather than a bacteriolytic mechanism.