Aggregation of Blood Cells by

• A rapid, reproducible method of determin-iEg the presence and relative amounts of adhesive and aggregated blood elements in the blood has recently been developed in this laboratory.1-2 The method involves measuring the pressure required to force blood at a con-stant rate through a standardized screen with multiple openings 20 micra square and 20 micra deep. This pressure has been named "screen nitration pressure. " During storage of blood1 and during exsanguination, plate-lets and leucocytes form aggregates which cause remarkable increases in this pressure. It has also been observed that adenosine di-phosphate; a component of platelets, causes platelets suspended in plasma to aggregate, and that monoamine oxidase inhibitors inhibit platelet aggregations. These observations have led to an interest in the composition of the platelets, and to further studies of the effects of certain components of platelets on the suspension stability of blood. One of these constituents, 5-hydroxytryptamine (ser-otonin), has become particularly interesting in this respect. Serotonin exerts a strong contractile influ-ence on certain smooth muscles, including the muscular layers of small blood vessels. In this paper, we wish to describe another striking effect of small amounts of 5-hydroxytrypta-mine—a very significant in vitro and in vivo adhesiveness and aggregation of platelets, leucocytes, and red blood cells resulting in a high screen nitration pressure, and slowin