We investigated the efficacy of amino acids 55-76 of the synthetic shrimp anti-lipopolysaccharide factor peptide (SALF(55-76) cyclic peptide), the C-terminal part of the shrimp anti-lipopolysaccharide factor. This study was conducted to elucidate the effects of the antiseptic action of this peptide. The SALF(55-76) cyclic peptide was tested against bacterial clinical isolates and showed broad-spectrum antimicrobial activity. Transmission electron microscopic (TEM) examination of SALF(55-76) cyclic peptide-treated Pseudomonas aeruginosa showed that severe swelling preceded cell death and breakage of the outer membrane; the intracellular inclusion was found to have effluxed extracellularly. When mice were treated with the SALF(55-76) cyclic peptide before bacterial challenge with P. aeruginosa, the peptide highly protected mice against death by sepsis. The P. aeruginosa recovered from SALF(55-76) cyclic peptide-treated mice after 4 h exhibited reduced bacterial growth similar to that recovered from vancomycin-treated mice. In addition, the syntheses of inflammatory cytokines, such as interleukin (IL)-2, IL-4, IL-10, IL-12, IL-13, interferon-gamma, and tumor necrosis factor [TNF]-alpha, were significantly upregulated 4 h after SALF(55-76) cyclic peptide treatment except for IL-4 in the liver. The expressions of Toll-like receptor 4 (Tlr4), Irf3, myd88, and Tram, were considerably elevated, but only Tlr4 existed in the spleen 4 h after SALF(55-76) cyclic peptide treatment. The prophylactic administration of SALF(55-76) cyclic peptide was begun the TNF-alpha response in comparison to untreated mice by an ELISA analysis. Due to its multifunctional properties, the SALF(55-76) cyclic peptide may become an important prophylaxis against and therapy for bacterial infectious diseases, as well as for septic shock.