1. The preparation from completely coagulated hemoglobin of crystalline soluble hemoglobin is described.
2. This soluble hemoglobin by all the tests tried has been indistinguishable from normal native hemoglobin which has never been coagulated.
3. The coagulation of hemoglobin is probably reversible.
4. Since hemoglobin is a typical coagulable protein, protein coagulation in general is probably reversible.