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Borosilicate glass (G) and commercial poly(methyl methacrylate) (PMM) surfaces were examined at pH 7, monovalent ionic strength 0.14 and 22 degrees C. Thrombin concentrations ranged from 6 X 10(-9) to 5.2 X 10(-8)M. Decreases in thrombin concentration in 10 ml vessles were determined by clotting time assay. Adsorption to PMM vessels is small. These were examined to establish the validity of the method. For G-vessels, the terminations of initial rapid decreases, associated with adsorption equilibrium, are accounted for mainly by an average isotherm having an association constant of 1.8 X 10(7) l./mol. Adsorption equilibrium is accompanied by a slower rate of decrease consistent with the surface denaturation of 2.5%/min of adsorbed molecules. Molecules remaining adsorbed to the surfaces of 1 mm i.d. G-capillaries were examined using the sequence of thrombin adsorption, buffer washing for times tw, removal of buffer and introduction of a fibrinogen aliquot for determination of effective surface thrombin concentration by a capillary clotting time. Most adsorbed molecules (less than 90%) essentially desorb within tw = 2 min, a second class (greater than 8%) desorb in tw similar to 30 min, and members of additional classes are still present after tw = 1440 min. For all of the properties examined, there is a small variance between vessels taken at the same time from the same batch, and a larger variance between groups of vessels taken at different times from the same batch or from different batches.