Abstract: Provided herein is a one-step method for chelating actinium-225 to a construct comprising a chelator linked to a bio-molecule, such as, an antibody or monoclonal antibody, via a bifunctional ligand in, for example, a 3-arm configuration. Also provided are methods for increasing the radiochemical yield of an actinium-225-chelant-biomolecule complex and for producing a high specific activity actinium-225 complex. The chelation is performed at a physiological temperature, about 37° C. Also provided are high specific activity actinium-225 complexes, that is, actinium-225 chelated to the chelator-biomolecule construct and pharmaceutical compositions thereof. Further provided are methods of treating a neoplastic disease or disorder with the actinium-225 complexes.