Glycosaminoglycan (GAG)-protein complexes from human plasma were separated into low charge (LC-GP) and high charge (HC-GP) components. LC-GP and HC-GP differed with respect to GAG and protein composition and to molecular size. The in vitro interaction of both GAG-protein complexes with human LDL was investigated. LC-GP did not precipitate LDL. On the contrary, HC-GP formed insoluble complexes with LDL, following a biphasic behaviour on increasing HC-GP concentration. In the presence of a HC-GP/LDL ratio higher than 0.02 the interaction stoichiometry was shifted towards the formation of soluble complexes. Papain treatment of HC-GP completely prevented LDL precipitation. Moreover, the extent of HC-GP-induced precipitation of LDL was markedly reduced by the simultaneous addition of LC-GP. Data obtained with standard GAGs showed that heparin (HE) and chondroitin-6-sulphate (C6S) were the most effective ligands in precipitating LDL. However, the shape of precipitation curves was markedly different. C6S behaved similarly to HC-GP, suggesting that GAG chains could play an important role in insoluble complex formation with LDL. Steady-state fluorescence anisotropy investigation indicated that HC-GP induced a significant decrease in the microviscosity of LDL hydrophobic region. This effect was no longer detectable after either addition of LC-GP or papain treatment of HC-GP. Differential scanning calorimetry (DSC) demonstrated that both lipid and protein components of LDL were affected by the interaction with HC-GP. The temperature of irreversible thermal unfolding of apo B100 was shifted to a lower value and a second peak appeared in the region of the reversible melting of cholesterol esters. Both the fluorescence anisotropy and the DSC data obtained with standard HE and C6S indicated that GAG chains were directly involved in affecting physico-chemical properties of complexed LDL. These results suggest that the interaction with plasma HC-GP could modify LDL structural properties. However, LC-GP is likely to act as a modulator, probably preventing the interaction between HC-GP and circulating LDL.