Sensitive methods to detect the earliest forms of colorectal cancers remain a challenge despite the development of serum and stool biomarkers. We reasoned that fluorescent affinity ligands derived from library screens can be developed to improve the detection and localization of early malignant lesions by endoscopy. We have developed an imaging agent for real-time endoscopic tumor detection in a murine model using a previously identified phage library-derived colon cancer-specific cyclic peptide and fluorescent moieties. The modified peptide had a 24 minute blood half life and tumoral accumulation was 6.9% of injected dose/g, approximately 7-fold higher than a scrambled control peptide. Orthotopic colonic tumors (HT29) were readily detectable by fluorescence endoscopy even when tumors were submucosal. These results show proof-of-principle that disease-specific library-derived fluorescent probes can be rapidly developed for use in the early detection of cancers by optical means.