We have studied monolayers of apolipoproteins Cl and All at the air/water interface, observing them along their isothermal compression processes with Brewster angle microscopy and using grazing incidence X-ray diffraction. The lateral order in Langmuir-Blodgett transferred proteins was also studied using atomic force microscopy (AFM). Previous studies have shown that APO CI consists of a chain with two amphiphilic alpha-helix motifs, whereas APO All is formed by two identical polypeptide chains, bonded by a disulfide bridge at position 6, where each chain consists also of two amphiphilic alpha-helix motifs. Monolayers of these proteins exhibit a first-order phase transition between two condensed phases at relatively high values of the lateral pressure and at room temperature. AFM images and X-ray diffraction peaks show that the phase transitions correspond to a phase change from a two-dimensional disordered phase to a more ordered state, where an unusual solid phase is discovered. Here, in each protein chain, one alpha-helix is confined to the interface, whereas the other alpha-helix is tilted toward the hydrophobic air.