To elucidate abnormalities in lipid metabolism in patients with primary biliary cirrhosis (PBC), plasma lipoproteins and apoproteins were analyzed in 10 such patients. Lipoprotein X was present in sera from five of the patients. Another abnormal lipoprotein of slow α-mobility on agarose gel electrophoresis was observed in sera of eight of the patients. The slow α-lipoprotein was distributed in the range of densities between low density and high density lipoproteins and was rich in apoprotein E. This abnormal lipoprotein of PBC was observed in those in Stages II and III but not in those in Stage I. The amounts of slow α-lipoprotein correlated to the levels of serum apoprotein E and to total cholesterol. Determination of apoprotein concentrations in serum of PBC patients revealed increases of apoproteins E and C-II and a decrease of apoprotein A-II. In conclusion, lipoprotein abnormalities in PBC patients were characterized by increased levels of apoprotein E and appearance of an abnormal lipoprotein of slow α-mobility in addition to well-known lipoprotein X.