In this study, we report the synthesis and the evaluation as MRI contrast agent of arabinogalactan/pyrrolidinoxyl radicals (PCA) covalent adduct (SLAG:Spin Labelled ArabinoGalactan). Arabinogalactan was used as targeting device, as it is recognized by the asialoglycoprotein receptor specific to the hepatocytes. The higher relaxivity R1 in water of SLAG, compared with small hydrophilic nitroxyl radicals, was explained by the molecular dynamics study using EPR spectroscopy that showed some immobilization of the radical into the polysaccharide. A binding study on isolated hepatocytes revealed that SLAG still recognizes the asialoglycoprotein receptor. MR imaging was performed using spin-echo T1 weighted images on mice to compare the contrast effect obtained with SLAG and PCA after IV injection (1 mmol/kg free radical). The percent signal enhancement observed in the liver 5 min after IV injection was 40 +/- 3% and 13 +/- 5% for SLAG and PCA, respectively. The signal was also dramatically increased in the renal cortex. This latter effect as well as the prolonged duration of the contrast (+/- 3 h), indicates at least a partial nonselective biodistribution; the high concentration needed to obtain a contrast effect could account for the saturation of the asialoglycoprotein receptor and hence for the apparent nonselective biodistribution.