We prepared a photoresponsive polypeptide, M(n) = 10 630, composed of two amphiphilic alpha-helical copolypeptides, poly[(gamma-methyl L-glutamate)-co-(L-glutamic acid)], joined by an azobenzene (am.-MAzoM) as a molecular photoreceptor. Dipalmitoylphosphatidylcholine monolayers containing the am.-MAzoM were formed at the air-water interface. We investigated photoinduced changes in the structure of the polypeptide-lipid monolayer systems. The alpha-helical rod of am.-MAzoM was laid at the air-water interface when the monolayer was in a liquid state. While in the solid condensed monolayer, the polypeptide molecules arranged perpendicular to the membrane (the air-water interface) and formed a bundle by self-association. The bundle of am.-MAzoM could be observed as an intramembranous particle ca. 4 nm in diameter with atomic force microscopy. UV light irradiation induced the destabilization of the bundle structure in the monolayer. This behavior arose from the photoinduced denaturation of the amphiphilic character of the polypeptide offing to a bending of the polypeptide in the main chain by a cis-tl ans photoisomerization of the azobenzene moiety. After removal of the light, the am.-MAzoM in the solid condensed monolayer returned to the original bundle structure.