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High-resolution ultrasonic imaging of circulating blood was used to study the relation between red cell aggregation and blood clotting in vitro. A reversible increase in echogenicity produced by red cell aggregation occurred in moving heparinized blood as shear rate was decreased. We induced clotting of the heparinized blood by administration of protamine. At both low (1.6 sec-1) and moderate (22.6 sec-1) mean shear rates, transient homogenous increased echogenicity indicative of red cell aggregation preceded blood clotting. In separate experiments, we established that protamine can cause increased echogenicity due to red cell aggregation which can be reversed by adding heparin to circulating suspended red cells in the absence of clotting factors. Presumably, these effects of protamine and heparin are due to electrostatic bonding involving red cell surfaces. We conclude from these studies that red cell aggregation precedes clotting of heparinized blood by protamine at low and moderate shear rates.