HP (2-20) (AKKVFKRLEKLFSKIQNDK-NH2) is an antibacterial 19-mer peptide derived from the N-terminal region of Helicobacter pylori ribosomal protein L1 (RPL1). Several truncated peptides were synthesized to investigate the effects of the N- or C-terminal regions of HP (2-20) on antimicrobial activity. The antimicrobial activity of the peptides was measured by their growth inhibitory effect upon Pseudomonas aeruginosa, Salmonella typhimurium, Saccharomyces cerevisae, Trichosporon beigelii and Candida albicans. Antimicrobial activity required a full length N-terminus. None of the peptides exhibited hemolytic activity against human erythrocyte cells. The membrane-disrupting activity of these peptides, using liposomes and 1, 6-diphenyl- 1,3,5-hexatriene (DPH) as a probe, confirmed that the full N-terminal region of HP (2-20) is a prerequisite for antibiotic activity and that this region may facilitate penetration of the cell membrane. Circular dichroism indicated that the alpha-helical structure of the peptides important for antimicrobial activity.