Abstract: The NGR peptide is a ligand specifically binding to tumor angiogenic blood vessels, and thus has potential usage in the diagnosis and therapy of tumor. In this study, the cyclic peptide YGGCNGRC is chemically modified by conjugation with a low molecular weight monomethoxy polyethylene glycol (mPEG, M. W. 1 900 or 5 000). They are labeled with 131 I by using the Iodogen method . The biodistribution results in normal mice and mude mice bearing MCF-7 breast carcinoma show that ~ 131 I-PEG 1900 -YGGCNGRC and 131 IPEG 5000 -YGGCNGRC have lower uptake in most organs and tissues, but higher renal uptake than ~ 131 I-YGGCNGRC . Their early uptake in blood is lowered and their retention kept approximately the same as ~ 131 I-YGGCNGRC . The deiodination from ~ 131 I-PEG_ 5000 -YGGCNGRC is significantly lower than from ~ 131 I-YGGCNGRC . Although the tumor uptake of ~ 131 I-PEG 5000- YGGCNGRC is a little bit less than ~131 I-YGGCNGRC , the uptake ratio of tumor to normal tissue is increased for most examined organs. The tumor/muscle ratio is increased from 3.5±1.7 to 5.7±2.2. Modification of cyclic NGR peptide with mPEG does not remarkably alter the tumor/blood ratio.