The potential of the stable isotope 26Mg (natural abundance 11.01 atom %) as a biological tracer has been explored using neutron activation (NA) and mass spectrometry (MS) for detection. The high natural abundance of this isotope restricts its utility to purposes that can be met by means of an oral tracer. Net magnesium absorption can be measured by administration of 50-60 mg 26Mg and subsequent measurement of fecal isotope excretion. True absorption can be estimated by making isotope measurements in urine or plasma when a dose of oral 26Mg is supplemented by i.v. administration of 28Mg. Although NA analysis is feasible, it is time-consuming and relatively insensitive. MS analysis of the volatile chelate Mg (2,2',6,6'-tetramethyl-3,5-heptanedione)2 (Mg(THD)2) is more sensitive in measuring 26Mg enrichment and requires smaller absolute amounts per replicate analysis. This method is also time-consuming, however, and confounded by instrumental memory effects. Future uses of 26Mg will depend on the development of automated MS instruments utilizing thermal or other means of direct ionization of mineral elements, and/or innovative approaches to the use of biological tracers. Possibilities are in vitro methods that can substitute for in vivo absorption tests of intrinsically labeled foods, and cells and tissues cultured in close to 100% 24Mg to reduce the high abundance of 26Mg in natural magnesium sources.