1. Serum labelled with radioactive iron was adjusted to different degrees of iron saturation and clearance of the tracer from the circulation of rats and its uptake by liver and intestine were followed.
2. The results showed that: (a) plasma iron clearance did not vary according to whether labelled iron was attached to transferrin at high or low iron saturation; (b) uptake of labelled iron by the liver was greater from transferrin at high iron saturation than from transferrin at low saturation.
3. The results suggest the presence of a receptor mechanism in the liver similar to that previously shown in reticulocytes.