1. The cataphoretic velocity of blood platelets (horse) in plasma has been found to be between 40 and 51 µ per sec. per volt per cm. The mean velocity obtained from five horses is . 45 µ per sec. per volt per cm.
2. The cataphoretic velocity of polymorphonuclear leucocytes in similar specimens is practically identical with that of the platelets. This is noteworthy because of the fact that lymphocytes and red cells have different speeds.
3. With spontaneous agglutination of platelets, white cells and red cells, there is no change in the cataphoretic velocity incidental to aggregation.
4. The possible surface composition of platelets and white cells is briefly discussed.
5. The bearing of these findings on the origin of blood platelets and the mechanism of thrombus formation is demonstrated.