Some physical properties of the mixtures composed of esterified and free cholesterol at various ratios were measured. Addition of a small proportion of free cholesterol, lowered slightly the melting point of the esterified cholesterol fraction from human plasma and it was kept nearly constant at this level by the addition of 30-40% of free cholesterol. Further addition of free cholesterol resulted in an abrupt rise in the melting point. The ranges of the stable plateau were clearly wider in esterified cholesterol with more polyunsaturated acids. The extent of inhibition of monolayer formation of free cholesterol with cholesterol palmitate was found to be greatest at a mixture ratio of 60-70%. In the case where cholesterol palmitate was added to an equimolar mixture of dipalmitoyl glycerophosphocholine and free cholesterol, a trimolecular mixed monolayer was formed above the mixing ratio, 7.5:2.5, of esterified and free cholesterol. The present studies clearly showed that there were distinct changes in some physical properties examined at the molar ratio of esterified cholesterol:free cholesterol of about 7:3.