Three different but related abnormal lipoprotein species, LP-X1, LP-X2, and LP-X3, have been isolated from cholestatic plasma by ethanol precipitation and zonal ultracentrifugation. All three populations are rich in phospholipids (64.9 to 67.5%) and cholesterol (23.0 to 26.8%) but poor in cholesteryl esters (0.4 to 1.9%), triglycerides (1.8 to 3.2%), and protein (3.2 to 6.7%) with differences in chemical composition which result in buoyant densities (1.038, 1.049, and 1.058, respectively) to allow their separation. LP-X1, LP-X2, and LP-X3 exhibited apparent flotation rates of 17.3, 9.7, and 3.2 Svedbergs and Stokes radii of 339, 343, and 294 A, respectively. As determined from circular dichroic measurements, the protein constituents of all three particles possessed a high degree of alpha helical structure (41 to 65%). Each LP-X particle exhibited abnormally low fluidity as evaluated by electron paramagnetic resonance. All of the particles contained human serum albumin and the C-proteins as major protein constituents, but only LP-X2 and LP-X3 contained apolipoprotein A-I and apolipoprotein E.