LL-23 is a natural peptide corresponding to the 23 N-terminal amino acid residues of human host defense cathelicidin LL-37. LL-23 demonstrated, compared to LL-37, a conserved ability to induce the chemokine MCP-1 in human peripheral blood mononuclear cells, a lack of ability to suppress induction of the pro-inflammatory cytokine TNF-α in response to bacterial lipopolysaccharides (LPS), and reduced antimicrobial activity. Heteronuclear multidimensional nuclear magnetic resonance (NMR) characterization of LL-23 revealed similar secondary structures and backbone dynamics in three membrane-mimetic micelles: SDS, dodecylphosphocholine (DPC), and dioctanoylphosphatidylglycerol. The NMR structure of LL-23 determined in perdeuterated DPC contained a unique serine that segregated the hydrophobic surface of the amphipathic helix into two domains. To improve our understanding, Ser9 of LL-23was changed to either Ala or Val on the basis of homologous primate cathelicidins. These changes made the hydrophobic surface of LL-23 continuous and enhanced antibacterial activity. While identical helical structures did not explain the altered activities, a reduced rate of hydrogen-deuterium exchange from LL-23 to LL-23A9 to LL-23V9 suggested a deeper penetration of LL-23V9 into the interior of the micelles, which correlated with enhanced activities. Moreover, these LL-23 variants had discrete immunomodulatory activities. Both restored the TNF-α dampening activity to the level of LL-37. Furthermore, LL-23A9, like LL-23, maintained superior protective MCP-1 production, while LL-23V9 was strongly immunosuppressive, preventing baseline MCP-1 induction and substantially reducing LPS-stimulated MCP-1 production. Thus, these LL-23 variants, designed on the basis of a structural hot spot, are promising immune modulators that are easier to synthesize and less toxic to mammalian cells than the parent peptide LL-37.