The nature of the encephalopathic agent present in monkey bone marrow has been investigated. It is readily soluble at a pH of 7.4, as well as at the low pH of 2.0 and 3.0. The point of minimal solubility is pH 4.2. Heat inactivation is produced by a temperature of 75°C., acting for 15 minutes. The agent is digested by pepsin. It exhibits a slight degree of sedimentation with the air-driven centrifuge. It is non-dialyzable. It is precipitated from solution (pH 7.4) by ammonium sulfate, over a range of 0.4 to 0.8 saturation. These findings all suggest that the agent is a protein. No immunological reactions could be elicited.

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