In this work we report the structural characteristics of bovine serum albumin/poly(ethylene glycol) lipid conjugate (BSA/PEG(2000)-PE) complexes under physiological conditions (37 degrees C and pH 7.4) for particular fractions of BSA to PEG-lipid concentration, c(BSA)/c(PEG)(2000)-PE. Ultraviolet fluorescence spectroscopy (UV) results shown that PEG(2000)-PE is associated to BSA, leading to protein unfolding for fixed c(BSA) = 0.01 wt % and variable c(PEG)(2000)-PE = 0.0015-0.6 wt %. Tryptophan groups on the BSA surface are in contact with the PEG-lipid at c(PEG)(2000)-PE = 0.0015 wt %, while they are exposed to water at c(PEG)(2000)-PE > 0.0015 wt %. Dynamic and static light scattering (DLS and SLS) and small-angle neutron scattering (SANS) point out the existence of individual BSA/PEG-lipid complexes in the system for fixed c(BSA) = 1 wt % and variable c(PEG)(2000)-PE = 0.15-2 wt %. DLS shows that there is only one BSA molecule per protein/PEG-lipid complex, while SLS shows that the PEG-lipid associates to the BSA without promoting aggregation between adjacent protein/polymer-lipid conjugate complexes. SANS was used to show that BSA/PEG(2000)-PE complexes adopt an oblate ellipsoidal shape. Partially unfolded BSA is contained in the core of the oblate ellipsoid, which is surrounded by an external shell containing the PEG(2000)-PE.