Antimicrobial peptides (AMPs) exhibit cell selectivity and activity against microorganisms and are promising candidates as pharmaceutical agents. In mammalian cell membranes cholesterol plays a regulatory function in antibiotic drug resistance and the immune response. Our hypothesis is that differences in the peptide-membrane interactions versus cholesterol affect the bilayer properties giving a possible framework for selectivity of AMPs for bacterial membranes. Here we employed solid-state 2H NMR to compare the degree of softening or stiffening by AMPs compared to cholesterol in model di-monounsaturated phosphatidylcholine and phosphatidylethanolamine lipids (DOPC vs. DOPE). Solid-state 2H NMR quadrupolar-echo experiments and relaxometry experiments [1] investigated changes in structural properties and mechanical properties due to the membrane-peptide interaction. The DOPC and DOPE with LL37 (2-4 mol%) model systems mimicking mammalian and bacterial membranes were studied with POPC-d31 (10 mol%) as a probe lipid. The residual quadrupolar couplings from quadrupolar-echo experiments yielded segmental order parameters (SCD) for the individual acyl segments. Notably LL37 decreased the quadrupolar splittings indicating it requires a specific area per lipid, while cholesterol showed the opposite behavior. The model-free functional dependence of spin-lattice relaxation rates on SCD (square-law plots) indicated greater bending rigidity with cholesterol for both DOPC and DOPE membranes. By contrast LL37 decreased the bending rigidity of the membranes. Additionally bilayers comprising POPE/DMPG/cardiolipin (90:5:5), and DMPC:cholesterol (90:10) were modeled with LL37 peptides on the bilayer surface. All-atom molecular dynamics simulation trajectories captured the molecular-level interactions. We observed partial insertion of the LL37 in the POPE system, while the peptides were repelled from the membrane in the DMPC/cholesterol system. These observations reveal the surprising rivalry between cholesterol and AMPs which underlies AMP mechanism and selectivity. [1] T.R. Molugu et al. (2016) Chem. Phys. Lipids 1848, 246.