Apolipoprotein F (ApoF), one of the minor apolipoproteins in human plasma, has been recently isolated and partially characterized [Olofsson, S.O., McConathy, W.J., & Alaupovic, P. (1978) Biochemistry 17, 1032-1036]. In the present work, the interaction of ApoF with other apolipoproteins and lipids in human plasma was studied. By the successive use of immunosorbers specific for ApoF, apolipoprotein A-II (ApoA-II) and apolipoprotein A-I (ApoA-I), three different ApoF-containing lipoproteins were isolated from normolipidemic fasting human plasma. Their apolipoprotein content was determined by double immunodiffusion against monospecific antisera to all known serum apolipoproteins, electroimmunoassay, crossed immunoelectrophoresis, and polyacrylamide gel electrophoresis. Their lipid composition was determined by thin-layer chromatography. The three ApoF-containing lipoproteins were identified as LpF:A-I:A-II (lipoprotein containing ApoF, ApoA-I, and ApoA:II), LpF:A-I (lipoprotein containing ApoF and ApoA-I), and LpF (lipoprotein containing only ApoF). LpF:A-I:A-II was found to contain ApoF, ApoA-I, and ApoA-II in an apparent 2:1:1 molar ratio. Its lipid moiety was characterized by cholesterol ester (45%) and free cholesterol (28%) as the predominant lipids. LpF contained only ApoF, and in its major lipid components were also cholesterol esters (63%) and free cholesterol (21%). It is suggested that ApoF-containing lipoproteins may be involved in transport and/or esterification of cholesterol.