Expression of the epithelial-specific integrin A v B 6 is low or undetectable in most adult tissues but may be increased during wound healing and inflammation and is up-regulated dramatically by many different carcinomas, making A v B 6 a promising target for the in vivo detection of cancer using noninvasive imaging. In addition, A v B 6 is recognized as promoting invasion and correlates with aggressive behavior of human cancers and thus agents that recognize A v B 6 specifically in vivo will be an essential tool for the future management of A v B 6 -positive cancers. Recently, we identified the peptide NAVPNLRGDLQV-LAQKVART (A20FMDV2), derived from foot-and-mouth disease virus, as a potent inhibitor of A v B 6 . Using flow cytometry and ELISA, we show that this peptide is highly selective, inhibiting A v B 6 -ligand binding with a IC 50 of 3 nmol/L, an activity 1,000-fold more selective for A v B 6 than for other RGDdirected integrins (A v B 3 , A v B 5 , and A 5 B 1 ). A20FMDV2 was radiolabeled on solid-phase using 4-[18 F]fluorobenzoic acid, injected into mice bearing both A v B 6 -negative and A v B 6 -positive (DX3puro/DX3puroB6 cell lines) xenografts and imaged using a small animal positron emission tomography (PET) scanner. Rapid uptake (<30 min) and selective retention (>5 h) of radioactivity in the A v B 6 -positive versus the A v B 6 -negative tumor, together with fast renal elimination of nonspecifically bound activity, resulted in specific imaging of the A v B 6 -positive neoplasm. These data suggest that PET imaging of A v B 6 -positive tumors is feasible and will provide an important new tool for early detection and improved management of many types of cancers. [Cancer Res 2007;67(16):7833-40]