Phage displaying an Arg-Gly-Asp (RGD)-containing peptide with a high affinity for αv integrins homed to tumors when injected intravenously into tumor-bearing mice. A substantially higher amount of αv-directed RGD phage than control phage was recovered from malignant melanomas and breast carcinoma. Antibodies detected the αv-directed RGD phage in tumor blood vessels, but not in several normal tissues. These results show that the αv integrins present in tumor blood vessels can bind circulating ligands and that RGD peptides selective for these integrins may be suitable tools in tumor targeting for diagnostic and therapeutic purposes.