The B72.3 monoclonal antibody was radiolabelled with ¹²³I, and with ¹¹¹In and ⁶⁴Cu, using DTPA and SarAr, respectively. Their biodistribution in tumour-bearing nude mice was used to calculate the dosimetry of their respective therapeutic analogue, using ¹³¹I, ⁹⁰Y, ⁶⁷Cu, and ⁶⁴Cu. Two dosimetry models were used: one using the classical approach and a second model that takes into consideration the chemical stability of the radiolabelling methods employed and the biological clearance of each radioimmunoconjugate. Results clearly show that the ⁶⁴Cu-SarAr-B72.3 could be used as a therapeutic agent and, theoretically, be at least as effective as any of the other therapeutic radionuclides currently studied, such as ¹³¹I, ⁹⁰Y, and ⁶⁷Cu.