Molecular imaging is a powerful non-invasive tool that can be used for the identification and prognostic assessment of cancers, as well as the monitoring of therapeutic delivery and effect. Identification of novel biomarkers that can be used to specifically target tumours is of particular interest for imaging purposes. Tumour Endothelial Marker 8 (TEM8) is a recently identified cell surface protein associated with cell migration and adhesion 1 that appears to be specifically up-regulated during tumour angiogenesis. TEM8 overexpression has been identified on numerous tumours including colorectal carcinoma, 2 melanoma, 3 breast cancer, 4 non-small cell lung cancer, 5 Lewis lung carcinoma, 6 and osteosarcoma 7 but is undetectable on normal healthy tissues. Thus the tumour specific overexpression of TEM8 could be used as a potential imaging biomarker for diagnostic or therapeutic purpose. A candidate for targeting TEM8 is the naturally occurring ligand Protective Antigen (PA), a protein produced by the bacterium B. anthracis for targeted translocation of toxins into cells. 8 Utilizing the biological machinery of PA has potential to target therapeutics to tumour cells, 9 but there is a lack of in vivo data to support the assumption of tumour specificity. To assess the in vivo tumour specificity of PA we have produced recombinant PA and conjugated the protein to imaging molecules for optical and positron emission tomography imaging techniques. Studies were performed using a HPV38E6E7.FVB mouse model which produces spontaneous squamous cell carcinoma in response to repeated UVB irradiation. PA shows rapid, highly specific uptake and low background signal resulting in clear identification of tumours. The prolonged tumour retention and rapid background clearance suggests that PA would be a promising imaging agent capable of targeting a wide range of tumours.