The use of radioactive tracer elements in ion uptake assays allows the calculation of km, Ki and Vmax and determines the initial change in the ion content of the cells. Mg decays by the emission of a high-energy beta or gamma particle, which can be measured using a scintillation counter. However, the radioactive half-life of Mg, the most stable of the radioactive magnesium isotopes, is only 21 hours. This severely restricts the experiments involving the nuclide. Also, since 1990, no facility has routinely produced Mg, and the price per mCi is now predicted to be approximately US$30,000. The chemical nature of Mg is such that it is closely approximated by few other cations.  However, Co, Mn and Ni have been used successfully to mimic the properties of Mg in some enzyme reactions, and radioactive forms of these elements have been employed successfully in cation transport studies. The difficulty of using metal ion replacement in the study of enzyme function is that the relationship between the enzyme activities with the replacement ion compared to the original is very difficult to ascertain.