For a long time it has been vaguely realized that there is a relationship between food and blood formation. This idea has recently been put on a quantitative basis by the work of two groups of investigators: Whipple and his associates, by their experimentation on dogs, have determined the amount of hemoglobin formed in two-week periods from various types of food substances; Minot and his colleagues have established certain objective criteria for the effect of dietary substances in human anemias, mainly by the use of the reticulocyte response. If a substance has a positive effect on blood formation, increased numbers of young (reticulated) red blood cells appear in the circulating blood within ten days. This phenomenon has the obvious advantage of giving a rapid method of determining whether the substance administered to the patient is or is not effective. Anemia can presumably be produced either by an excessive loss or