The disappearance of free cholesterol in incubated serum is inhibited in some lightly heparinized subjects. This inhibition is accentuated when an emulsion of soy bean phosphatides is added to the serum prior to incubation. The combined effect ofin vivoheparin andin vitrophosphatide is the reverse of that produced by added phosphatide alone in unheparinized serum. In a limited number of experiments, the results with crystalline lecithin resembled those produced by soy bean phosphatides, but were more pronounced.