Previously we have reported that ultrasmall superparamagnetic iron oxide (USPIO) particles migrate across capillary endothelium, a prerequisite for the design of particulate pharmaceuticals for MR receptor imaging. In the current study, USPIO particles are directed specifically to asialoglycoprotein (ASG) receptors by coupling galactose terminals in the form of arabinogalactan (AG) to these particles. Biodistribution data showed that ASG-directed, AG-coated USPIO (AG-USPIO) particles selectively accumulate in the liver but not in other organs. Electron microscopy of liver showed electron-dense iron oxide particles bound to hepatocyte cell-surface membranes and in large numbers within intracellular lysosomes. The specificity of AG-USPIO for asialoglycoprotein receptors was confirmed by incubation experiments with and without ASG-blocking agents such as D(+)galactose and asialofetuin. In vivo MR imaging in rats showed a significant decrease in liver signal intensity at low doses (2 mumol Fe/kg); no significant changes were observed in the spleen. This decrease in signal intensity is larger than that observed with conventional iron oxides at equal doses. These initial data suggest that, for the first time, superparamagnetic agents can be directed to specific sites for MR imaging by strategies such as receptor targeting.