A radioimmunoconjugate composed of the F(ab')2 fragment of human monoclonal antibody PGN650 against phosphatidylserine (PS) labeled with the radioisotope iodine I 124, with potential imaging activity upon positron-emission tomography (PET). Upon administration, the MoAb moiety of PGN650 binds to exposed PS on tumor cells, thereby allowing for the visualization of tumors upon PET. This may facilitate the assessment of the effectiveness of antitumor agents. The phospholipid PS is normally located on the inner leaflet of the plasma membrane of healthy cells but is flipped to the outer leaflet in the endothelial lining of the tumor vasculature and other tumor cells in response to chemo- or radio- treatments in addition to oxidative stress.