In this study, N-methacryloyl-l-phenylalanine (MAPA) containing poly(2-hydroxyethylmethacrylate) (HEMA)-based magnetic poly(HEMA-MAPA) nanobeads [mag-poly(HEMA-MAPA)] were radiolabeled with I-131 [I-131-mag-poly(HEMA-MAPA)], and the radiopharmaceutical potential of I-131-mag-poly(HEMA-MAPA) was investigated. Quality control studies were carried out by radiochromatographic method to be sure that I-131 binded to mag-poly(HEMA-MAPA) efficiently. In this sense, binding yield of I-131-mag-poly(HEMA-MAPA) was found to be about 95-100%. In addition to this, optimum radiodination conditions for I-131-mag-poly(HEMA-MAPA) were determined by thin-layer radiochromatography studies. In addition to thin-layer radiochromatography studies, lipophilicity (partition coefficient) and stability studies for I-131-mag-poly(HEMA-MAPA) were realized. It was determined that lipophilicities of mag-poly(HEMA-MAPA) and I-131-mag-poly(HEMA-MAPA) were 0.12 +/- 0.01 and 1.79 +/- 0.76 according to ACD/logP algorithm program, respectively. Stability of the radiolabeled compound was investigated in time intervals given as 0, 30, 60, 180, and 1440min. It was found that I-131-mag-poly(HEMA-MAPA) existed as a stable complex in rat serum within 60min. After that, biodistribution and scintigraphy studies were carried out by using albino Wistar rats. It was determined that the most important I-131 activity uptake was observed in the breast, the ovary, and the pancreas. Scintigraphy studies well supported biodistribution results. Copyright (c) 2013 John Wiley & Sons, Ltd.