Lipid-protein interactions are of fundamental importance in the structure of biological membranes and of plasma lipoproteins. From previous studies [1-3] it seems reasonable to assume that the interactions between phospholipids and lipoprotein-protein (apoprotein) constituents are fundamental to the binding of neutral lipid by the plasma lipoproteins. For example, there is an insignificant binding of cholesteryl ester by the apoproteins of human HDL* in the absence of phospholipids. We have recently presented studies describing the probable location of phospholipid-binding site (s) in the MN-glycoprotein of the human red cell membrane [4-6] and of plasma lipoproteins [7-12]. Phospholipid-binding regions do not appear to be uniformly distributed along the length of the polypeptide chain [7-11, 13]; certain fragments of apoLP-Ala, apoLP-Gln-I and apoLP-Gln-II preferentially bind phospholipid as corn-* Abbreviations: VLDL, very low density lipoproteins; HDL, high density lipoproteins; apoprotein, a lipid-free protein component from a lipoprotein; apoLP-Ala and apoLP-Ser, two apoproteins from human VLDL, with carboxyl-terminal alanine and serine, respectively; apoLP-Gln-I (Al), and apoLP-Gln-II (A-II), the two major apoproteins of human HDL, each with carboxyl-terminal glutamine.