Innate immunity, especially that involving macrophage function, reportedly diminishes with advancing age and in patients with Alzheimer's disease (AD). In this study, we tried to elicit the non-specific activation of peripheral macrophages by oral administration of the herbal medicine Juzen-taiho-to (JTT), to assess its effect as a possible treatment for AD patients. Amyloid-beta protein precursor transgenic mice were used as a model of AD to clarify the effect of JTT. Activated macrophages derived from bone marrow cross the blood-brain barrier, and then develop into microglia, which phagocytose aggregated amyloid-beta (Abeta) in senile plaques. Here we show that orally administered JTT increased the number of CD11b-positive ramified microglia in the mouse brain. The immunohistochemical examination of brain sections stained with polyclonal anti-Abeta antibody showed reduced Abeta burden, and Abeta levels were also decreased in the insoluble fractions of brain homogenates, as determined by ELISA. Thus, the activation of peripheral macrophages by JTT might be a potential new therapeutic strategy for AD.