A case is reported of thrombocytopenia developing suddenly in a patient with metastatic carcinoma. Studies of this patient’s serum revealed a platelet agglutinin presumed to be related to the development of the thrombocytopenia in spite of the failure to demonstrate autoagglutination. In addition, a technique is described for the detection of a platelet lysin and clot retraction inhibitor, both of which could be demonstrated in this patient’s serum. The identity of the lysin and retraction inhibitor is suggested.

Studies on the serums of other patients showed a dissociation between the occurrence of platelet agglutinin on the one hand and of platelet lysin and retraction inhibitor on the other.

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