One of the characteristic pathological hallmarks of Alzheimer's disease (AD) are neuritic plaques that consist of amyloid peptide (Aβ). To improve diagnosis and treatment evaluation, neuroimaging tools that make use of Aβ-binding ligands to visualise amyloid plaques are being developed. We investigate the in vitro and in vivo characteristics of a series of three D-enantiomeric peptides (D1-D3) that were developed to specifically bind amyloid β1-42 (Aβ42) in the brains of transgenic AD-model mice. We stained brain sections of the mice, injected and infused the mice with these small D-peptides, and examined their staining of Aβ42 in the brain. The experiments demonstrate that the D-peptides label all plaques that contain Aβ42 in the brain. In contrast, diffuse amyloid β deposits (which do not contain Aβ42) are not stained by any of the D-peptides. The in vivo and in vitro studies demonstrate that the D-peptides label all Aβ42 in the brain, and none of the D-peptides causes inflammation or is taken up by astrocytes or microglia. Furthermore, long-term infusion of the pep- tides does not cause inflammation. Together, this demonstrates that these D-peptides might be suitable for use as molecular probes to measure Aβ plaque load in the living brain for early diagnosis of Alzheimer's disease, or to monitor Aβ42 plaque load during disease progression or during treatment. © 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.