The bioavailability and metabolism of different chemical species of mineral nutrients in the diet are receiving much attention from research nutritionists. In order to make scientifically based recommendations for mineral intakes, the chemical form of the mineral, with its specific absorption, utilization and retention, needs to be considered. Selenium is an example of an essential nutrient that is consumed in several different chemical forms, hence information is needed on the bioavailability and metabolism of each form before recommendations for dietary intakes can be made. A valuable tool for research on bioavailability and metabolism in humans is stable isotope tracers. When there are more than two stable isotopes available, as with selenium, stable isotope methodology allows the comparison of the utilization of different chemical forms of the nutrient simultaneously in in vivo studies. As an example of speciation questions addressed by nutritionists, a study is described that simultaneously evaluated utilization (absorption, retention and appearance in milk and blood) of two different chemical forms of selenium (selenite and selenomethionine) in lactating, non-lactating and never pregnant women using stable isotope tracers. All three groups of women had similar selenium status at the start of the study. Significantly more selenium from selenomethionine than from selenite was absorbed and appeared in the plasma in all groups. Milk contained more selenium from apparently absorbed selenomethionine than from selenite. All groups retained significantly more selenium from selenomethionine than from selenite; lactating women retained more selenium from selenite than did the other two groups, suggesting that milk losses may be partially compensated by enhanced retention of dietary selenium as selenite. Absorption and retention of selenium from selenomethionine in lactating women did not appear to be different from the other groups. The different chemical forms of selenium are metabolized differently among different physiological groups of women.