1. Heparin has been shown to increase platelet aggregation by ADP and adrenaline and to enhance the platelet release reaction when tested in citrated platelet-rich plasma (P.R.P.). This activity is present when heparin is added to P.R.P. or when P.R.P. is prepared after intravenous injection of heparin, and when heparin is added to non-anticoagulated native P.R.P.
2. Retention of platelets by cellophane membranes within a specially designed test-cell was significantly increased when heparin was added to citrated whole blood.
3. Though aspirin blocks the release reaction with and without heparin, it does not prevent the potentiation of initial ADP or first wave adrenaline aggregation caused by heparin.