We have identified the phase behavior of phosphoinositol (PI) lipid extracts from bovine liver and wheat in dioleoylphosphatidylcholine (DOPC) model membranes under physiological conditions (pH 7.4) and show, for the first time, that the physicochemical properties of phosphatidylinositol lipids are capable of driving changes in membrane curvature. Ten mole percent phosphoinositol (PI) extract in DOPC is sufficient to induce the formation of the inverse hexagonal (H-II) and inverse micellar cubic (Fd3m) phases at 37 degrees C. The phase behavior of several hydrated lipid samples was analyzed using small-angle X-ray scattering, and their lattice parameters were calculated.