Interactions between membranes and amyloid proteins are believed to be a major factor contributing to pathogenesis in amyloid diseases. Furthermore, membranes have been shown to closely affect fibrillation processes of varied amyloidogenic peptides. Here we describe an intriguing phenomenon in which bilayer-induced fibrillation of human calcitonin (hCT) gave rise to significant inhibition of membrane interactions of alamethicin, an antibiotic, membrane-permeating peptide. This "membrane shielding" effect was apparent only when fibrillation of hCT occurred in the presence of cholesterol-containing vesicles; no interference with membrane binding was detected when hCT fibrillar species were formed in noncholesterol lipid environments, or when hCT amyloid aggregates were separately added to lipid bilayers. The experimental data indicate that. cholesterol-promoted formation of amyloid fibril network at the bilayer interface is most likely responsible for the shielding effect. This phenomenon might point to a role of amyloid fibers in preventing membrane disruption by antibiotic peptides and other toxic species.