1. A patient with hemorrhagic diathesis associated with a prolonged clotting time is presented.
2. The clotting defect was corrected by normal and hemophiliac blood and normal platelet-containing plasma but was not corrected by idiopathic thrombocytopenic blood or by normal and leukemic platelet-poor plasmas as measured by the prothrombin consumption test.
3. The clotting defect appears most likely to be due to platelets which are defective in that area of platelet activity concerned with the activation of thromboplastinogen to form thromboplastin.