Plasma and red cell iron turnover rates were determined in 18 normal growing swine by the use of tracer doses of Fe59. Body surface counting was performed on a representative group of animals following the injection of the isotope.
The mean half-time of plasma iron disappearance was 1.19 ± 0.26 hours. Two exponential rates of disappearance of Fe59 from the plasma were observed in 15 of the pigs and a single rate was observed in the other 3. The mean plasma iron turnover rate was 1.11 ± 0.34 mg./kg. day. The average maximum incorporation of Fe59 into the erythrocytes was 92 ± 9 per cent. The mean red cell iron incorporation rate was 1.10 mg./kg. day, but an average of 0.42 mg./kg. day of this was calculated as being due to increasing red cell mass incidental to body growth, so that the true mean red cell iron turnover rate was 0.59 ± 0.19 mg./kg. day. The average "apparent" red cell life span was 63 ± 16 days. This is in agreement with the red cell life span of 62 days determined previously with glycine-2-C14.