1. The consumption of coagulation Factors IX, X, and XI was studied in normal whole blood clotted with celite (intrinsic activation) or tissue thromboplastin (extrinsic activation).

2. During these studies an assay method for Factor XI was developed which was not influenced by the presence of tissue thromboplastin. The assay method is based on the thromboplastin generation principle.

3. When blood clotted in silicone treated tubes, the serum and plasma concentrations of Factors IX, X, and XI were almost identical, indicating that little consumption or activation of these factors had occurred.

4. In the presence of celite, coagulation Factors IX and XI are consumed, whereas Factor X is consumed only slightly.

5. In the presence of tissue thromboplastin, Factor X is consumed, whereas Factors IX and XI are not consumed.

6. In the presence of both celite and thromboplastin, the thromboplastin decreased the consumption of Factors IX and XI produced by celite.

7. The study of serum coagulation factor levels may provide evidence as to whether the coagulation process had been initiated by the intrinsic (foreign surface contact) or extrinsic (thromboplastin) pathways.

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