It has long been known to those who have studied the variations in hemoglobin in the normal subjects that by far the greatest variations are those due to age. Our real knowledge of the age curve of hemoglobin begins with the work of Leichtenstern, who in 1878 published his results in a monograph entitled "Untersuchungen über den Hemoglobingehalt des Blutes in gesunden und kranken Zuständen." This author recognized very clearly that a precise knowledge of the variations due to physiologic conditions, especially age and sex, was an indispensable prerequisite for the study of the hemoglobin in diseased conditions. Leichtenstern's work was all done with the then newly discovered Vierordt's method, which was by far the most accurate method then available. This research extended into the estimation of the hemoglobin in a very large number of pathologic conditions, and the study of the variations due to age and sex was undertaken