The activity of the solubilized receptor was assayed by precipitating the receptor with acetone in the presnce of egg phosphatidyleholine liposomes. It was found that the binding of ~(125)I-LDL to fetal liver receptors rose significantly with the increase of fetal age. During the same period, the total cholesterol (TC) and LDL-eholesterol (LDL-C) levels in fetal serum declined progressively. The correlation between fetal age and concentration of TC and LDL-C were -0.80(P