Human apolipoproteins All, CIII, E3, and AI were studied from different points of view: secondary structure prediction analysis, protein stability to overcome and revert thermal denaturation, and phase behavior of their monolayer. We found a well-preserved relationship among the predicted secondary structure, amphipatic cl-helices, and the ability to recover their secondary protein structure after thermal treatment. With each one of the apolipoproteins studied, the equilibrium character of the unfolding process and the presence of isochromatic points suggested a two-state character for the process. Pressure-area isotherms were carried out, as well as direct observations with a Brewster angle microscope of the apolipoproteins monolayers deposited onto a highly ionic water subphase. We described the gas/liquid-phase transition in all of these proteins. The apolipoproteins Al and All present a phase transition between two condensed phases at high lateral pressures. A model of the secondary structure for these proteins is presented.