Proton-enhanced 13C nuclear magnetic resonance is used to obtain signals from labeled cholesterols in lecithin dispersions. The [26-(13)C]cholesterol resonance indicates that the aliphatic tail of the molecule undergoes reorientation fast enough to average completely the chemical shift anisotropy. In contrast, [4-(13)C]cholesterol signals are characteristic of limited anisotropic reorientation. The resonances from the 4 position are sensitive to the temperature-concentration phase diagram. A phase boundary is observed at about 20 mole percent cholesterol.