Monoclonal antibodies for radioimmunotargeting are often tested in tumour bearing nude mice. In vivo determination of the uptake of the monoclonal antibody in the tumour requires quantitative scintigraphy, and this in turn requires an adequate method for subtraction of radiation from the normal tissue. For this reason, two different methods for background subtraction were evaluated, a contralateral background region of interest or an irregular one, surrounding the tumour. A pinhole collimator was used for the scintigraphy and the monoclonal antibodies were labelled with 125I. Furthermore, a method was developed for estimation of the mean absorbed dose in the tumour from these repeated quantitative scintigraphic measurements. This requires that the tumour mass can be accurately estimated in vivo. Finally, the results were compared with in vitro measurements of the uptake.