The relative brightness of adult white matter in T1-weighted MRI arises from myelin, but the mechanisms responsible remain to be clarified. Koenig et al. [Magn. Reson. Med. 14, 482 (1990)] conjectured that the cholesterol of myelin (approximately 30% of its lipid) was responsible. We present 1/T1 and magnetization transfer contrast imaging data [Wolff and Balaban, Magn. Reson. Med. 10, 135 (1989)] on a model system--50% lipid--50% water by weight, with the lipid one-half phosphatidyl choline (PC) and one-half cholesterol--and a control in which the lipid is all PC. The differences between the model and control samples mimic the myelin contribution to white matter in both experiments.