The alpha-particle-emitting radionuclide 223Ra (t(1/2) = 11.4 d) is of interest for use in targeted radionuclide therapy. In order to provide radium-labeled monoclonal antibodies, the development of a chelator binding radium in a stable fashion is required. As a part of the search for potentially useful radium chelators, the relative stability of 223Ra-chelates with linear and cyclic chelating agents was evaluated by means of competition extraction experiments.