For safe therapy with radiolabelled antibodies it is essential to maximise the uptake of radionuclide in the tumor compared with normal tissue. This paper outlines the methods and limitations of establishing the dosimetry before administering the therapeutic dose. Whole body distributions and kinetics are established using quantitative radionuclide scanning techniques, single photon tomography and blood counts in conjunction with images from other modalities such as x-ray CT. Our experience with about 40 therapeutic cases, involving mostly intracavity administration, will be outlined.