A cubic monoacylglycerol-protein-water phase has been identified by low-angle X-ray diffraction, and the main features of the ternary phase diagram monoolein/lysozyme/water are presented. The thermal stability of the protein in the lipid-protein cubic phase has been examined by differential scanning calorimetry. According to the physical properties of the phase it is proposed that the protein molecules are located in the water medium, i.e. in the water channel systems of the cubic structure earlier suggested. The ability of various proteins to form this cubic phase has been studied, and it was found that the formation of this phase is favoured by an isoelectric point (pI) far from pH 7 in a salt-free solution, thus by high electrostatic repulsive forces.