Poly(L-lysine) bound to phosphatidylglycerol or phosphatidic acid bilayers was submitted to hydrostatic pressure in a diamond anvil cell to investiage whether the lipidic surfaces can protect the polypeptide against pressure-induced conformational transformations. The amide I region of the infrared spectrum of dimyristoylphosphatidic acid bound polylysine shows that most of the polypeptide retains its -sheet structure up to 19 kbar, while it is known to convert entirely to α-helix at ~2 kbar in the absence of the lipid [Carrier, D., Mantsch, H. H., & Wong, P. T. T. (1989) Biopolymers (in press)]. The simultaneous binding of the polypeptidic molecules to two opposing bilayers appears to be required in order to preserve the β-sheet structure at pressures over ~9 kbar: a small proportion of the polypeptide, most likely the molecules at the surface of the aggregated bilayers, was found to convert to unordered and eventually to α-helical conformations in the pressure range 9-19 kbar. The decrease from 1612 to 1606 cm⁻¹ of the frequency of the major β-sheet component of the infrared amide I band as the pressure is raised to 6 kbar indicates a strengthening of the interchain hydrogen bonds. The high-pressure infrared spectra of polylysine bound to dimyristoyl- and dipalmitoylphosphatidylglycerol show that the polypeptide remains α-helical up to ~12 kbar, though the changes in the bandshape indicate an increase in hydrogen bond strength. The formation of a small amount of β-sheet was observed during decompression and is attributed to the effect of dehydration on the polypeptidic molecules located at the surface of the aggregates. This study suggests that lipidic surfaces could be used as cocatalysts, to control and protect the active conformation of enzymatic proteins for operation at elevated hydrostatic pressure.