SUMMARY The present study was designed to investigate the cellular uptake of technetium‐99m (99mTc) and chromium‐51 (51Cr) by autoradiographic methods and to compare these data to those obtained by gamma scintillation counting. HeLa cells, a fibrosarcoma of murine origin (Sarcoma I) and human peripheral blood lymphocytes were labelled with either 99mTc‐sodium pertechnetate or 51Cr‐sodium chromate. Cells were non‐uniformly labelled with both radionuclides. Nevertheless, uptake as determined by both autoradiography and gamma scintillation counting revealed that each cell type had a characteristic labelling pattern which appeared to have a relationship to cell surface area. This was determined to be linear for 99mTc when counts per minute were plotted as a function of area (μm2). Grains were randomly scattered over the entire cell surface and occasionally tracks could be seen as a trail radiating out from the periphery. HeLa cells had the largest surface area, the highest grain counts and the greatest uptake of both 99mTc and 51Cr as determined by gamma counting. Lymphocytes had the smallest surface area, the lowest grain counts and the least uptake. Both radionuclides are non‐specific cell labels which are taken up independently of mitosis and protein synthesis. The favourable chemical properties of 99mTc‐sodium pertechnetate have led to its use as a label for a wide variety of inorganic and organic compounds which are employed as radiopharmaceuticals. Autoradiographic methods could be useful in determining the cellular localization of these compounds, as well as any protein molecule with which pertechnetate forms chelates. Although the 6 h half life of 99mTc limits its use to short term experiments, it does permit brief exposure times prior to development.