A method for measuring whole-body ⁵⁹Fe has been evaluated which employs longitudinal scan geometry, a unique arrangement of multiple NaI(Tl) detectors, and photon measurements in the Compton region. Radio-iron has been quantitated from a point source embedded in plastic pellets, from tracer localized in the gastrointestinal tract, and during movement of in vivo activity into and out of the erythroid marrow. In all instances, counting levels varied by no more than 2%. Measurements have also been made for two weeks following oral and intravenous radio-iron administration, to define the day-to-day reproducibility of counting and to establish the validity of measuring radio-iron absorption and losses by this technique. The suitability of the method for measurements of iron assimilation is demonstrated.