text: A number of chelators of divalent cations have different fluorescence spectra in the bound and unbound states. Chelators for Ca are well established, have high affinity for the cation, and low interference from other ions. Mg chelators lag behind and the major fluorescence dye for Mg (mag-fura 2) actually has a higher affinity for Ca. This limits the application of this dye to cell types where the resting level of Ca is  is to be measured. Recently, Otten et al. (2001) have described work into a new class of compounds that may prove more useful, having significantly better binding affinities for Mg. The use of the fluorescent dyes is limited to measuring the free Mg. If the ion concentration is buffered by the cell by chelation or removal to subcellular compartments, the measured rate of uptake will give only minimum values of km and Vmax.