Secondary ion mass spectrometry (SIMS) microscopy is the only method potentially capable of mapping all the elements in the periodic table including stable and radioactive isotopes. We used this method to study 99mTc distribution by detecting and localizing of 99Tc, a daughter product which has the same mass and the same chemical properties. It was combined with albumin macroaggregates or with Hexamethylpropylene-amine oxime (HMPAO) in leukocytes. The efficiency of 99Tc ionization under Cs+ bombardment was higher than with an O2+ beam. By using high mass resolution we succeeded in detecting and localizing 99Tc in cell sections by eliminating polyatomic ions that arise from this biological matrix. The 99Tc specific signal was obtained with a mass resolution of 2000 for labeled albumin macroaggregates, and 5000 for HMPAO-labeled leukocytes. In the latter, the labeling varied from one cell to another and 99Tc was present in both the nucleus and the cytoplasm. The results indicate that SIMS microscopy can provide new insights into 99mTc dosimetry.