1. 59Fe-labelled ferric chloride was introduced into tied loops of guinea-pig jejunum.
2. After 5–30 min the loop was removed, the enterocytes were isolated, homogenized and subjected to analytical subcellular fractionation.
3. Uptake of 59Fe was extremely rapid and after 5 min 45% of the radioactivity sedimented in the mitochondrial fraction.
4. Density gradient centrifugation indicated that approximately 80% of this radioactivity was associated with the mitochondria themselves; the remainder was in brush-border fragments.
5. Selective disruption of the mitochondria demonstrates that the iron is localized to the matrix and the inner membrane, indicating transport of the absorbed iron into the organelle.
6. It is suggested that mitochondria are actively implicated at an early stage in the intestinal transport of iron.