Stimuli-responsive nanocarriers based on lipid self-assemblies have the potential to provide targeted delivery of antimicrobial peptides, limiting their side effects while protecting them from degradation in the biological environments. In the present study, we design and characterize a simple pH-responsive antimicrobial nanomaterial, formed through the self-assembly of oleic acid (OA) with the human cathelicidin LL-37 as model for an amphiphilic antimicrobial peptide. Colloidal transformations from core-shell cylindrical micelles with a cross-section diameter of ~ 5.5 nm and a length of ~ 23 nm at pH 7.0, to aggregates of branched thread-like micelles at pH 5.0 were detected using synchrotron small angle X-ray scattering, cryogenic transmission electron microscopy, and dynamic light scattering. Biological in vitro assays using an Escherichia coli bacteria strain showed high antimicrobial activity of the positively charged LL-37/OA aggregates at pH 5.0, which was not caused by the pH conditions themselves. Contrary to that, negligible antimicrobial activity was observed at pH 7.0 for the negatively charged cylindrical micelles. The nanocarrier's ability to switch its biological activity 'on' and 'off' in response to changes in pH could be used to focus the antimicrobial peptides' action to areas of specific pH in the body. The presented findings contribute to the fundamental understanding of lipid-peptide self-assembly, and may open up a promising strategy for designing simple pH-responsive delivery systems for antimicrobial peptides.