We have developed a one-step procedure to introduce both polyethylene glycol (PEG) and the metal chelator diethylenetriaminepentaacetic acid (DTPA) to proteins through a heterofunctional PEG precursor. The PEG precursor contains DTPA at one end and an amine-reactive isothiocyanate (SCN-) functional group at the other end. It was obtained as lyophilized powder and could be stored at 4 degrees C for several months. Protein conjugation was achieved by simply mixing the proteins and the PEG precursor SCN-PEG-DTPA in an aqueous solution. As exemplified by the PEGylation and radiolabeling of annexin V, the resulting conjugate 111In-DTPA-PEG-annexin V showed selective binding to apoptotic cells in vitro and increased blood half-life in vivo. The PEGylated, radiolabeled annexin V may be useful in the noninvasive imaging of apoptosis.