Low-density lipoproteins (LDL) in plasma are constructed from a single molecule of apolipoprotein B-100 (M(r) 512 000) in association with lipid (approximate M(r) 2-3 x 106). The gross structure was studied using an updated pulsed-neutron camera LOQ with an area detector to establish the basis for the interpretation of structural changes seen during dynamic studies of LDL oxidation. Neutron-scattering data for LDL in 100% 2H2O buffers emphasize their external appearance. Guinier analysis on a continuous-flux neutron camera D17 revealed pronounced concentration-dependences in the radius of gyration, R(G), and the intensity of forward scattering, I(0) (equivalent to the M(r) of LDL) between 0.5 and 11 mg of LDL protein/ml. LDL preparations from different donors gave different R(G) values. When extrapolated to zero concentration, R(G) values ranged between 8.3 and 10.6 nm and were linearly correlated with M(r), which is consistent with a spherical structure. The distance-distribution function P(r) in real space showed a single maximum at 9.1-10.9 nm, which is just under half the observed maximum dimension of 23.1 ± 1.2 nm expected for a spherical structure. The neutron radial-density function ρ(r) exhibited a plateau of high and featureless density at the centre of LDL. LDL can be modelled by a polydisperse assembly of spheres with two internal densities and a mean radius close to 10.0 nm in a normal distribution of radii with a standard deviation of 2.0 nm. The data are consistent with recent electron-microscopy and ultracentrifugation data. Although D17 and LOQ gave similar results, the short acquisition times for full scattering curves obtained with LOQ at LDL concentrations close to physiological are advantageous for oxidation time-course studies of structural changes, and no radiation damage is observed. Structural changes during LDL oxidation are best studied at a concentration of 2 mg of LDL protein/ml or higher, and data on non-oxidized LDL are an important control.