The serum of a patient diagnosed as a primary biliary cirrhosis was studied during the various evolutive stages of the disease. During the non-icteric period, the serum lipoprotein had the chemical composition (rich in cholesterol and phospholipids) of that found in cholestasis; however the bilirubinemia was normal, and no LPX was detected; there was a normal esterified to total cholesterol ratio and the alphaLP had not decreased, as expected, but increased and was divided into two fractions; immunoelectrophoretic studies of ultracentrifugal fractions showed some LP with HDL immunological properties, but with lower densities than that of normal HDL. We suggest that the phospholipid overloading of some of these HDL in the patient's serum could explain the changes in density. Some months after this study, the patient's serum exhibited the characteristics found in cholestasis, with disappearance of HDL and appearance of LPX. During the subsequent period, the serum again contained phospholipid rich HDL and LPX. In fact several LPX with different densities were found. It seems, therefore, that in cholestasis, LP undergo modification in which the HDL peptide chains play an important role.