A CONVENIENT way of obtaining white cells from whole blood is simply to allow EDTA-blood to settle in siliconized glasses and then pipette off the leucocyte-rich supernatant after 1.5 h or more. A more rapid separation occurs when agents which aggregate erythrocytes are added to the blood1. The effect of these agents may be utilized even without mixing them with the blood. From physical chemistry it is known that a reaction between two compounds may be increased manifold if it occurs at an interface2. Similarly, the clumping of the erythrocytes may be accelerated when the process occurs at an interface.