Previously, two D-enantiomeric amino acid peptides, D1 and D3, which specifically bind to the amyloid-beta peptide Abeta(1-42), were identified by phage display selection. To assess the diagnostic and therapeutic potentials of D1 and D3 for the diagnosis and treatment of Alzheimer disease, the blood-brain barrier transport of these D-peptides was quantitatively evaluated in vitro. Our results showed that the apical-to-basolateral transport of D3 was more efficient than that of D1. An active efflux transport mechanism seems to oppose the transport of D1, whereas D3 is likely to be transported through the blood-brain barrier via an adsorptive-mediated transcytosis mechanism.