LP-X, a lipoprotein present in the low-density range (d 1.006--1.063 g/mL) of cholestatic human plasma, has been studied with its normal counterpart (LDL) by 1H and 31P nuclear magnetic resonance. The 220-MHz 1H spectrum of LP-X contains four major lines: the choline CH2N and N+(CH3)3 resonances and the cholesteryl--acyl CH2 and CH3 envelopes. The widths of these four lines at 37 degrees C are approximately 24, 10, 124, and 48 Hz, respectively. The latter two line widths are much greater than the corresponding ones of LDL (28 and 20 Hz), suggesting the much more restricted motion of acyl chains and/or cholesteryl rings in LP-X. This difference persists over the temperature range 15--52.5 degrees C. The microscopic fluidity of LP-X and LDL was compared by titration with 2,2,6,6-tetramethylpiperidinyl-1-oxy (Tempo), a paramagnetic amphiphile which distributes between the bulk aqueous phase and the fluid lipid phase of lipoproteins. Tempo is much less effective in broadening the 1H resonances of LP-X than of LDL, indicating the lower permeability/fluidity of the former. The 40.5-MHz 31P spectrum of LP-X consists of a single resonance whose line width is approximately 20 Hz and whose spin--lattice relaxation time is 2.23 +/- 0.15 s. Titration of LP-X with Pr3+ ions splits this resonance into two lines, one remaining at the chemical shift of the original resonance and the other paramagnetically shifting downfield. The ratio of integrated areas for these two lines was 1:1.72. Titration of phosphatidylcholine--cholesterol vesicles alone, vesicles containing apolipoprotein-C and albumin, or vesicles containing apolipoprotein-X gave results similar to those obtained with native LP-X, suggesting the presence of a single bilayer structure in all of these systems.