Antimicrobial peptides are small, usually alpha-helical peptides that kill cells by disrupting the cell membrane. Recently, we have been studying a simple 'designed' antimicrobial in simple, single component bilayers. Using a deuterated peptide and polarised neutron reflection, we have been able to monitor the orientation of the peptide in the membrane, which is an essential step in determining it's mechanism of action. To make progress in understanding the basis of selectivity of these peptides for prokaryotes however, it is necessary to understand how the behavioour of the peptide differs in bacterial case. Recently, we have demonstrated the fabrication of stable, asymmetric membranes which are intended as mimics of the gram-negative bacterial outer membrane. In this proposal, we wish to compare the orientation of the peptides in these membranes as compared to a eukaryotic mimic.