There is an urgent need for development of novel antimicrobial therapies, and the mammalian theta-defensins and protegrins seem to offer significant potential in this regard. These peptides are crosslinked by multiple disulfide bonds and - in the case of the defensins - are also cyclised through their N- and C-termini. Both exhibit potent antibacterial activity, but differ in the extent of their side effects in terms of red blood cell lysis. Neutron reflectivity studies will be performed to characterise the interaction between these two peptides and model bacterial cell membranes. The peptides for study include the Rhesus monkey theta-defensin, RTD-1, and the porcine protegrin, PG-1. Interaction of these two peptides with a model bacterial membrane will be characterised using a floating phospholipid bilayer supported above a gold-coated substrate with covalently bound thioalkyl monolayer