Thromboplastic preparations were obtained from rabbit brain. The data indicate that standard acetone-dried reagent functions by activating factor X, with factor VII as an accelerator. When crude tissue reagent was fractionated by butanol treatment and saline extraction, a water-soluble material was obtained which similarly activated factor X. In each case, a source of phosphatide was necessary for activated factor X to yield full clotting activity; this phosphatide was provided in the crude tissue reagent but it was lacking in the water-soluble extract.

The extract was further purified and characterized. It appears to be a protein and it evidently activates factor X enzymatically. The data thus indicate that brain thrombloplastin contains two separable clotting activities: one of these activates factor X, the other supplies phosphatide.

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