The usefulness of serum ferritin as a measure of subclinical stages of iron deficiency has been tested in monkeys by inducing a mild iron deficiency dietarily over a period of 12 months. Various biochemical indicators of iron status were measured periodically along with analysis of liver iron and iron staining of bone marrow samples obtained by biopsy, at the end of the experiment. A mild form of iron deficiency was confirmed by bone marrow staining for iron. Of all the biochemical indicators tested, significant decreases were seen in hemoglobin and hematocrit at the 11th and 10th months, respectively. These changes were consistent with the changes later found in bone marrow grading for iron. Serum ferritin concentration and liver iron concentration did not show any significant difference between the controls and iron-deficient monkeys. Thus, these results do not support the existence of a latent stage of iron deficiency. In the mild form of iron deficiency, the functional compartment, represented by bone marrow iron and hemoglobin, is sensitive to depletion even when there were no changes in storage compartment represented by liver iron and serum ferritin.