Small doses of iron (5 to 20 mg) were used to study absorption in normal and mildly iron-deficient men. Plasma iron concentration was measured before and at frequent intervals after ingestion of iron. Men with normal iron stores showed little change in plasma iron levels, while mildly iron-deficient men showed large increases. This consistent observation of increases in plasma iron levels indicates that the iron tolerance test using small doses of iron is a sensitive indicator of iron deficiency. It provides a means of studying iron absorption without the use of radioactive tracers.