Copper and iron are essential elements in the human organism, and recent enlightening research has given increased evidence of their importance in the blood stream. In our first experiments, nine years ago, on the iron content of whole blood, we found it necessary to establish a large series of determinations on normal persons.1 The average iron content of whole blood for 200 men was 50.13 ± 0.15 mg. per hundred cubic centimeters, and for 100 women it was 43.42 ± 0.19. The mathematical index, the mode, the figure which appears most frequently in the series of determinations and around which the majority of figures group themselves, was 50 mg. of iron per hundred cubic centimeters for 200 men and 45 mg. for 100 women.1b The fact that in women the average fell below the mode illustrates the tendency toward the anemic state in women. The determination of iron