The subendothelial surface of rabbit aorta and alpha chymotrypsin-digested subendothelium were exposed to anticoagulated human blood in an annular flow chamber. The wall shear rate was similar to that observed in large arteries (830 sec-1) and exposure times varied from 2 1/2 to 40 min. The platelet reactive substrate of alpha chymotrypsin-digested subendothelium consists of a three-dimensional meshwork of collagen fibrils which form islands of variable size and height in a matrix of virtually unreactive elastin. Collagen-induced aggregation in the aggregometer was similar with or without prior alpha chymotrypsin-digestion of a highly dispersed preparation of fibrillar collagen. The rate of platelet adhesion was decreased on the fibrallar collagen of alpha chymotrypsin-digested subendothelium as compared to intact subendothelium. On the other hand the rate of aggregation was increased once platelets adhered to the fibrillar collagen. Mural thrombi (aggregates) disappeared on subendothelium whereas they grew progressively on the fibrillar collagen. Thus the fibrillar collagen of alpha chymotrypsin-digested subendothelium appears to be a more thrombogenic surface. It is suggested that physical (loose three-dimensional meshwork versus a comparatively solid surface) and/or chemical (number of platelet reactive sites per unit surface area) differences between the two surfaces may explain the platelet-surface-interaction patterns which are characteristic for each surface.