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1. An investigation of the physicochemical properties of myosin has been carried out. Prepared under standard conditions, the ratio of flow-birefringence to protein concentration is uniform. The effect of electrolytes, pH, and urea on the flow-birefringence and viscosity (relative and anomalous) of myosin has been examined. 2. Decrease or abolition of flow-birefringence does not necessarily imply far reaching denaturation, since such effects can be reversed by a variety of means. 3. When a myosin solution is treated with adenosinetriphosphate, its flow-birefringence is decreased (average 48 per cent), its anomalous viscosity is retained, and its relative viscosity is decreased (average 14 per cent). The full effect of adenosinetriphosphate is obtained at 0.004 M; a molarity very much less than that of other substances which decrease the flow-birefringence of myosin. 4. The changes in the physicochemical properties of myosin brought about by adenosinetriphosphate are spontaneously reversible, and are connected with the enzymatic action of the protein as adenosinetriphosphatase. 5. Effects similar to those of adenosinetriphosphate on the physicochemical properties of purified myosin have been obtained so far only with inosinetriphosphate. 6. Inorganic phosphate is split off by myosin from inosinetriphosphate as well as from adenosinetriphosphate. Inorganic triphosphate is split by 1 to 2 per cent solution of three times precipitated myosin. 7. Adenosinediphosphate and inorganic triphosphate act as competitive inhibitors with adenosinetriphosphate, blocking the fall of flow-birefringence. 8. The implications of the results, and the conception of active enzymic groups attached to proteins participating in cell structure, whether contractile or non-contractile, are discussed in relation to present views on muscle physiology and other biological problems.