Three immunochemically and electrophoretically distinct lipoproteins, LP-A, LP-B, and LP-X,(1) were isolated from the low density lipoprotein fraction (1.006-1.063 g/ml) in plasma from patients with biliary obstruction by a separation procedure which combines ultracentrifugation, heparin precipitation, and ethanol fractionation. This method, here described, permits the quantitative determination of individual plasma lipoprotein families on the basis of their protein moieties, rather than on the basis of their lipid moieties or density. The chemical composition of the unique lipoprotein, LP-X, was similar to that of an abnormal lipoprotein, OLP, isolated by Russ et al. (29) and by Switzer (30). In obstructive jaundice plasma, the combined LP-X and LP-B accounted for 98% and the LP-A for only 2% of the total protein content of the LDL fraction. This study indicates that the plasma lipoprotein pattern in obstructive jaundice is characterized by (a) a decreased concentration of HDL, (b) an increased concentration of LDL, and (c) the presence in the LDL fraction of varying amounts of a specific lipoprotein, LP-X, immunochemically and chemically distinct from LP-A and LP-B. LP-X, with its characteristically high content of unesterified cholesterol and phospholipids, is primarily responsible for the unusual protein and lipid content of the LDL fraction. Screening tests in 61 patients with various forms of jaundice indicated that a characteristic immunoelectrophoretic precipitin are between plasma samples and purified antibodies to LP-X was observed only in patients with obstructive jaundice. This simple immunochemical test may represent a valuable new tool in the differential diagnosis of obstructive and nonobstructive jaundice.