A new technique was devised for interaction studies between human blood and chemically modified amorphous collagen surfaces. Blood is drawn directly from an antecubital vein, through saline-filled collagen hollow fibers, at either constant flow for varying time or constant time at varying flow. Platelets and fibrin were deposited more rapidly and in greater numbers on methylated surfaces and more slowly and to a lesser extent on succinylated surfaces than on native surfaces. Increasing blood flow was associated with increased fibrin formation. Adherent platelets spread over 3-8 mu of methylated surface, but retained a spherical shape (1-2 mu) on succinylated and native surfaces. Thus, charge modification of collagen significantly alters platelet adhesion, subsequent platelet morphology, and fibrin deposition. Methylated surfaces are thrombogenic and succinylated ones are relatively thrombo-resistant.