1. A procedure is described for isolating a copper protein from horse liver in pure form, in a state that appears to be crystalline.

2. This copper protein is colored blue-green, and its copper content varied between 0.3 and 0.4 per cent. No indication of any catalytic properties was obtained in the limited number of tests made. Because of its variable copper content, the protein may function for the storage of copper in the body.

3. The copper could be removed partly from the protein by dialysis against cyanide, and a colorless crystalline protein was obtained by lowering the pH to 5.0, followed by dialysis. The original blue-green color of the protein could be restored by dialysis against copper acetate solution.

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