In the study of hematological disorders there has been great enthusiasm for the use of radioactive isotopes as investigative tools, but their application to clinical diagnosis has not been so readily accepted. At present the general concensus is that vitamin B12 labeled with cobalt 60, erythrocytes labeled with chromium 51, and iron salts labeled with iron 59 should supply information of importance in the clinical classification of anemias. It seems timely, therefore, to inquire whether these isotopic techniques supply information which is either unique, more accurate, or more readily measured than are conventional diagnostic procedures. To this end the experience in this laboratory with these radioactive compounds is presented. The evidence would suggest the following conclusions: 1. The measurement of the intestinal absorption of cobalt 60 vitamin B12 supplies information regarding the lack of gastric intrinsic factor in pernicious anemia which is not readily obtained by other means.