During active blood regeneration in anemia in dogs an increase occurs in the stroma protein of the red cells. When vitamin B(12) with radioactive cobalt is given at the start of this blood regeneration one finds concentration of labeledB(12)in the stroma protein but not in the hemoglobin. After the acute phase of red cell regeneration is ended the concentration of B(12) in stroma protein falls rapidly to very low levels within 2 weeks. Subsequent episodes of red blood cell regeneration seems not to cause remobilization of radioactive cobalt into red cells from other body stores. It appears that the vitamin B(12) is a factor of importance in the first steps of stroma protein formation in the first few days of the life of the red cell in the dog. This response in dogs and the response in pernicious anemia to vitamin B(12) may have some points in common. Distribution of the B(12)-radioactive cobalt in the organs and tissues at autopsy has been recorded. Some very suggestive localizations were noted and some variation 1 week and 7 weeks after B(12) injections. Radioactive cobalt escapes in the urine during the weeks following B(12) injections.