1. The degree to which a tracer quantity of 55Fe bound to transferrin and introduced into the plasma 36 months previously had equilibrated with the body iron was studied in a subject who was phlebotomized repeatedly until iron-deficiency anaemia developed.
2. Analysis of the results with two mathematical models showed that equilibration with haemoglobin iron and storage iron was complete.
3. Estimates of the quantity of iron initially stored in the body by three different methods yielded values of 37·3, 33·1 and 34·8 mmol.
4. Since the previously determined mean initial plasma ferritin concentration was 185 μg/l (462 pmol/l), there was 0·179–0·197 mmol of stored iron per 1 μg of plasma ferritin/l.
5. The study suggests that the assumption on which currently accepted values for daily body iron loss were calculated is valid.