The method described by Helen Wright for estimation of the adhesiveness of platelets has been found to be reproducible. Adhesiveness of platelets is reduced in the presence of hypocoagulable blood. It may or may not be enhanced when the blood is hypercoagulable. In hemophilia, the adhesiveness of platelets has been found to become increased beyond the premedication level when the coagulation defect is corrected by Fraction 1 derived from human plasma. Two cases exhibiting bleeding are described in which deficient adhesiveness of the thrombocytes was the only demonstrable defect noted in the coagulation mechanism.