Lateral compressibilities of bilayers in multilayer lattices are given for 10 phospholipid preparation:dilauryl-, dimyristoyl-, dipalmitoyl-, distearoyl-, and dioleoylphosphatidylcholine (PC); egg phosphatidylethanolamine (PE); as well as cholesterol-containing bilayers of dipalmitoyl PC or of egg PC. Bilayer deformability is highly nonlinear and does not permit description in terms of a simple modulus. The presence of cholesterol or C=C bonds (dioleoyl PC) increases deformability, but freezing of acyl chains does not cause dramatic stiffening of the bilayer. Lateral compression of dilauryl PC an dimyristoyl PC causes a transition from "melted" to "frozen" acyl chains above the normal transition temperatures. Our measurements do not correspond in any obvious way to lateral compressibilities in monolayers at the air-water interface.