Iron content of rat serum ferritin. 
The serum ferritin concentration was significantly higher in female than in male rats, reflecting higher iron stores in females than in males.  The mean iron/protein ratio of serum ferritin was 0.018+/-0.008 (SD) (microg of Fe/microg of protein) in female rats and 0.011+/-0.011 in male rats, being much lower than that of liver ferritin (0.233+/-0.014 in females and 0.227+/-0.020 in males).  Iron loading of rats significantly increased serum ferritin concentration, but did not influence the iron content of serum ferritin.  These results indicate that rat serum ferritin contains only a small amount of iron independent of body iron stores.