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Waddell, Steenbock, Elvehjem, and Hart1showed that rats fed milk exclusively developed nutritional anemia and that the ingestion of pure iron salts failed to cure it. However, normal regeneration of hemoglobin was obtained with small additions of copper sulphate along with the iron salt. These results have been confirmed by Keil and Nelson.2Myers and Beard3found that higher doses of zinc and magnesium retarded blood regeneration. If this is the case, anemia should develop more rapidly on milk and FeCl3if these elements are included. The slope of the curves obtained with and without zinc or magnesium demonstrate that anemia resulted as readily with or without these elements.Prior to this work on hemoglobin regeneration, Daniels and Hutton4demonstrated that rats failed to reproduce on milk and iron, but reproduction was a success when a mineral mixture of NaI, NaF, KAl (SO4)2, MnSO4, and Na2SiO3was added. They concluded that all of these salts were necessary in the milk for reproduction. The work of Hart, Steenbock and associates1and Keil and Nelson2is so conclusive, that it is certain animals cannot grow and reproduce normally without copper. There must have been contamination of this element either in the milk or salt mixture used by Daniels and Hutton. The minimum amount of copper required for regeneration of hemoglobin is extremely small. The minimum amount of copper as CuSO4, when injected intraperitoneally, is shown to be 0.002 mg. daily. The milk used in these experiments was collected directly into glass containers in order to avoid contamination with copper. Milk ad liband 0.5 mg. of Fe as FeCl3daily were fed to the rats. 0.05 mg. of Fe as FeCl3injected intraperitoneally with 0.002 mg. Cu as CuSO4daily does not result in normal regeneration.