The mole fraction of cholesterol in biomembranes varies from nearly zero to about 0.5. The principles that determine how much cholesterol can be accomodated in a biomembrane are not yet understood. A phenomenon with a relatively simple thermodynamic description is the maximum solubility of cholesterol in a bilayer, beyond which cholesterol precipitates as crystals. We were able to detect trace amounts of cholesterol monohydrate crystals coexisting with phospholipid-cholesterol lamellae. This sensitive detection enabled us to establish the phase boundary for appearance of the crystalline phase. The x-ray source at CHESS provided the resolution needed to distinguish higher order lamellar repeats from the low angle cholesterol monohydrate lines. The value of cholesterol solubility in (16:0,18:1[unreadable]9)-PC, (12:0,12:0)-PC, (22:1[unreadable]13,22:1[unreadable]13)-PC, and (16:0,18:1[unreadable]9)-PS is mole fraction 0.66. In (16:0,18:1[unreadable]9)-PE, the value is 0.50. The underlying molecular basis for this difference is not understood, but seems to involve headgroup area. However, a simple prediction is that the inner leaflet of cellular plasma membranes should contain less cholesterol than the outer leaflet, because of the nearly exclusive location of PE in the inner leaflet.