Two procedures for quantitative determination of the cholestasis-characterizing lipoprotein-X are described. Both procedures are based on ether-extract-ability of phospholipids which are relatively loosely bound in lipoprotein-X. The presence of cetyltrimethylammonium bromide at adequate concentration allowed complete ether extraction of phospholipids from lipoprotein-X even at low concentration, without affecting the phospholipids of normal lipoproteins. In one modification phospholipids were extracted into ether ion-paired with a bilirubin conjugate (e.g. Woodward's K conjugate) the concentration of which was then estimated in the ether extract. In the second modification phospholipids were directly determined in the ether extract. Calibration was done either with a lipoprotein-X concentrate or with a clear sonicated lecithin sol added to normal serum. The results obtained by both methods are in a good agreement with the results by agar gel electrophoresis.