1. Washed isolated platelets added to physiologic saline produce fall in surface tension.
2. Higher concentrations of platelets in saline cause a greater fall in surface tension than lower concentrations.
3. After centrifugation of platelets suspended in saline, the supernatant may show a fall in surface tension as compared to saline in which no platelets have been suspended. In the higher concentrations of platelets, the fall is much greater than in the lower concentrations.
4. When platelets are suspended in heparinized plasma, they exhibit either no change or a higher surface tension than do either their supernatants following centrifugation or the heparinized plasma alone.
5. No full explanation for the observed phenomena can be given.