Three approaches to calculating radiation absorbed doses and answering fundamental questions in radiolabelled antibody therapy are outlined. Conventional macroscopic dosimetry is probably not appropriate for calculations on the small scale of normal tissue and tumour volumes that are of interest in therapy. A better though even more traditional approach is to use the results from beta particle dosimetry. These may be shown to apply down to quite small populations of cells. An approach using the results found in microdosimetric work is probably too far from practical application to be useful at present.