Alzheimer's disease (AD) is associated with accumulation of β-amyloid (Aβ). A major genetic risk factor for sporadic AD is inheritance of the apolipoprotein (apo) E4 allele. ApoE can act as a pathological chaperone of Aβ, promoting its conformational transformation from soluble Aβ into toxic aggregates. We determined if blocking the apoE/Aβ interaction reduces Aβ load in transgenic (Tg) AD mice. The binding site of apoE on Aβ corresponds to residues 12 to 28. To block binding, we synthesized a peptide containing these residues, but substituted valine at position 18 to proline (Aβl2-28P). This changed the peptide's properties, making it non-fibrillogenic and non-toxic. Aβl2-28P competitively blocks binding of full-length Aβ to apoE (IC50 = 36.7 nmol). Furthermore, Aβl2-28P reduces Aβ fibrillogenesis in the presence of apoE, and Aβ/apoE toxicity in cell culture. Aβl2-28P is blood-brain barrier-permeable and in AD Tg mice inhibits Aβ deposition. Tg mice treated with Aβ12-28P for 1 month had a 63-3% reduction in Aβ load in the cortex (P = 0.0043) and a 59.5% (P = 0.0087) reduction in the hippocampus comparing to age-matched control Tg mice. Antibodies against Aβ were not detected in sera of treated mice; therefore the observed therapeutic effect of Aβl2-28P cannot be attributed to an antibody clearance response. Our experiments demonstrate that compounds blocking the interaction between Aβ and its pathological chaperons may be beneficial for treatment of β-amyloid deposition in AD.