Each subject will be placed on a fixed low-iron diet consisting of four rotating meal plans designed by our nutritionists in consultation with the subjects themselves. Twenty-four hour specimens of urine and stool will be collected daily and their iron content determined by atomic absorption. Net excretion will be determined taking into account the iron content of all uneaten food. The excretion of copper and zinc will also be monitored to insure that these physiologically important metals are not depleted. Each subject will be given a unit of blood on days 1, 11, 21 and 34. This will ensure that the level of erythropoiesis is approximately the same before each drug regimen.