Alzheimer's disease (AD) is one of the most devastating neurodegenerative disorders. The neuropathological hallmarks include extracellular senile plaques consisting of deposited β-amyloid (Aβ) peptides and intraneuronal neurofibrillary tangles. Neuroinflammation and activation of astrocytes are also well-established features of AD neuropathology; however, the relationships between astrocytes and Aβ deposition remain unclear. Previous studies have shown that adult mouse astrocytes internalize and degrade Aβ deposits in brain sections prepared from human amyloid precursor protein (APP) transgenic mice. In the present study, we demonstrate that cultured adult, but not neonatal mouse astrocytes, respond morphologically and degrade Aβ deposits present in human AD brain. We also transplanted astrocytes isolated from enhanced green fluorescent protein expressing adult and neonatal mice into the hippocampi of human Aβ plaque-bearing transgenic APPSwe+PS1dE9 (APdE9) mice and their wild-type littermates and followed the migration and localization of these astrocytes by confocal microscopy upto 7 days after transplantation. Posttransplantation the astrocytes localized as aggregates or thin strings of many cells within the hippocampi of APdE9 and wild-type mice and showed limited migration from the injection site. Interestingly, most of the transplanted astrocytes were found near Aβ deposits in the hippocampi of APdE9 mice. In contrast to findings in ex vivo degradation assay, confocal microscopy revealed that both adult and neonatal transplanted astrocytes internalized human Aβ immunoreactive material in vivo. These results support the role of astrocytes as active Aβ clearing cells in the CNS that may have important implications for future development of therapeutic strategies for AD. © 2007 Wiley-Liss, Inc.