Lipoproteins in normal and cholestatic human plasma were studied using crossed immuno-electrophoresis and electroimmunoassay. Crossed immuno-electrophoresis against anti-α-lipoprotein revealed a complex pattern of α-lipoproteins in cholestasis resembling that of α-lipoproteins in a patient with familial deficiency of the enzyme lecithin cholesterol acyl transferase. With anti-β-lipoprotein an absence of pre-β-lipoproteins with normal electropho-retic mobility was noted in crossed immunoelectrophoresis. These observations indicate that false values may be obtained with the use of electroimmunoassay for the determination of lipoprotein concentrations in cholestasis. A method employing crossed immunoelectrophoresis with an intermediate gel for the quantitative determination of lipoprotein X is proposed.