When one considers how incomplete is our knowledge of the anatomy and physiology of the hematopoietic system it is perhaps not remarkable that the diseases of that system should have remained so long shrouded in obscurity. Although the clinical pictures of the anemias and leukemias are sharply defined, their pathology still offers many problems. When in his classical description of pernicious anemia Addison noted that it occurred "without any discoverable cause," he stated the first and greatest problem of that disease and one which a halfcentury of investigation has not been able to solve, although by clinical, pathologic and experimental studies it has been able to narrow the field of search somewhat. However, this, the etiology of the disease, still remains its great problem. The second problem arose at a time when the clinical picture, especially the blood picture of this anemia described by Addison, had not been sharply defined,