Amyloid β (Aβ) immunotherapy is emerging as a promising disease-modifying therapy for Alzheimer's disease, although the precise mechanisms whereby anti-Aβ antibodies act against amyloid deposition and cognitive deficits remain elusive. To test the “peripheral sink” theory, which postulates that the effects of anti-Aβ antibodies in the systemic circulation are to promote the Aβ efflux from brain to blood, we studied the clearance of 125I-Aβ1-40 microinjected into mouse brains after intraperitoneal administration of an anti-Aβ monoclonal antibody 266. 125I-Aβ1-40 was rapidly eliminated from brains with a half-life of ∼30 min in control mice, whereas 266 significantly retarded the elimination of Aβ, presumably due to formation of Aβ-antibody complex in brains. Administration of 266 to APP transgenic mice increased the levels of monomer Aβ species in an antibody-bound form, without affecting that of total Aβ. We propose a novel mechanism of Aβ immunotherapy by the class of anti-Aβ antibodies that preferentially bind soluble Aβ, i.e., intracerebral, rather than peripheral, sequestration of soluble, monomer form of Aβ, thereby preventing the accumulation of multimeric toxic Aβ species in brains.