Magnesium-28 was applied to specific leaves of bean (Phaseolus vulgaris) and barley (Hordeum vulgare) plants. After 24 hours, as much as 7% of the absorbed Mg was exported from the treated bean leaves and 11% was transported basipetally from the treated zone of the barley leaves. Transport of Mg did not occur past a heat-killed section of the treated leaf, thereby indicating that translocation took place via the phloem. Mg movement in the phloem was also evident in autoradiograms of bean stem segments in which the xylem was separated from the phloem by a thin sheet of plastic.