We studied 4039 children who were 6 months to 12 years of age to characterize developmental variations of serum ferritin and erythrocyte protoporphyrin. Age-related descriptive statistics were derived. The -2 SD value for serum fettitin was found to increase progressively from 12 to 21 μg/l with increasing age, while the +2 SD value for erythrocyte protoporphyrin was found to decrease progressively from 65 to 42 μg/dl whole blood with increasing age. While the mean serum ferritin value was found to continue to rise throughout the first 12 years of life, erythrocyte protoporphyrin values were highest at 1 to 2 years of age, then fell to essentially constant levels after 4 to 6 years of age. These relationships, as well as the linear relationship of increasing hematocit and serum ferritin with increasing age, suggest that the rise of hematocrit with age, as previously observed, is associated with improving storage and availability of iron for heme synthesis.