The deposition of amyloid-β (Aβ) peptides into amyloid plaques precedes the cognitive dysfunction of Alzheimer's disease (AD) by years. Biomarkers indicative of brain amyloid burden could be useful for identifying individuals at high risk for developing AD. As in AD in humans, baseline plasma Aβ levels in a transgenic mouse model of AD did not correlate with brain amyloid burden. However, after peripheral administration of a monoclonal antibody to Aβ (m266), we observed a rapid increase in plasma Aβ and the magnitude of this increase was highly correlated with amyloid burden in the hippocampus and cortex. This method may be useful for quantifying brain amyloid burden in patients at risk for or those who have been diagnosed with AD.