The measurement of the106Ru content in99mTc eluates from99Mo-99mTc generators is difficult because of the proximity of the103Ru and106Ru gamma-ray spectral peaks and the low106Ru content.106Ru causes a relatively high whole-body dose, similarly as known contaminants60Co,134Cs,140Ba and some others. Assuming that ruthenium is eliminated from the body with the biological half-life 7.3 days (according to ICRP) the additional absorbed dose caused by106Ru does not exceed, with regard to the low content of this nuclide, the dose due to103Ru. The results are given of the preliminary experiment in animals with the aim to determine the biological behaviour of ruthenium radioisotopes after the administration of the99mTc eluate containing these contaminants.