Because the effects of red blood cell (RBC) concentration on hemostasis and thrombus formation have not been studied experimentally under conditions of whole blood flow without anti-coagulation, normal baboons were bled or transfused to obtain three different groups: a low hematocrit (Ht) group (20% less than Ht less than 25%), a normal Ht group (35% less than Ht less than 40%), and a high Ht group (50% less than Ht less than 55%). Measurements of platelet count, bleeding time, platelet aggregation, fibrinogen level, and coagulation time (APTT) were equivalent to normal values in each group. Thrombus formation was induced using a device composed of collagen-coated tubing followed by two sequentially placed expansion chambers designed to exhibit flow recirculation and stasis. The device was exposed for up to 40 minutes in an arterio-venous shunt system. Wall shear rates in the tubular collagen segment were 100 seconds-1 and 500 to 750 seconds-1. The accumulation of 111In-platelets and 125I-fibrinogen/fibrin was measured radioisotopically; RBC incorporation was determined from measurements of total thrombus hemoglobin. Thrombus that formed on the collagen substrate was rich in platelets and poor in fibrin and RBCs. Under high flow conditions, thrombus composition showed no dependence on Ht. Surprisingly, under low flow conditions, platelet thrombus volume was negatively correlated with Ht (r = -.73, P = .005), and was increased by greater than twofold in the low Ht group as compared with the high Ht group. Thrombus that formed in the disturbed flow regions contained relatively few platelets but was rich in fibrin and RBCs. The predominant finding was a positive correlation between RBC incorporation and Ht at both high and low shear rates (r = .90, P = .00003; and r = .77, P = .002, respectively), with thrombus volume increasing three- to sixfold between the low and high Ht groups. Thus, in vivo variations in Ht ranging between 20% and 55% did not affect hemostasis, but were found either to promote or inhibit the net accumulation of thrombus, depending on local flow conditions.