The plasma low density lipoproteins (LDL) in biliary obstruction are characterized almost exclusively by the presence of the immunochemically distinct lipoprotein families, lipoprotein B (LP-B) and lipoprotein X (LP-X). It is suggested that LP-X, with its uniquely high content of unesterified cholesterol and phospholipid, is primarily responsible for the unusual lipid composition of LDL and the abnormal plasma lipid composition in obstructive jaundice. To show their protein moieties, we isolated LP-X and LP-B from the LDL in plasma obtained from patients with obstructive jaundice. A separation procedure was employed which combines ultracentrifugation, heparin precipitation, and ethanol fractionation. Whereas LP-B was characterized by the presence of apolipoprotein B (ApoB), intact LP-X contained a protein moiety of unique composition consisting of a mixture of albumin (approximately 40%) and the specific apolipoprotein, ApoX (60%). These two protein moieties were separated by preparative ultracentrifugation at d 1.21 g/ml of a solution of partially delipidized LP-X. LP-X thus comprises an albumin-lipoprotein complex in which the masked antigenic site of albumin can be revealed by partial or total delipidization. Apolipoprotein X, the characteristic nonalbumin protein moiety of intact or partially delipidized LP-X, was immunochemically different from ApoA, ApoB, albumin, gamma-globulins, and other serum proteins. The results of analytical ultracentrifugation and the immunochemical and electrophoretic properties of ApoX indicated it to be a complex protein consisting possibly of several nonidentical polypeptides. ApoX was characterized by its amino acid composition, and by serine and threonine as the major N-terminal and alanine as the major C-terminal amino acids. It has been suggested that ApoX is similar to, if not identical with, apolipoprotein C.