SummaryAfter early failures, a series of injection molded Silastic 372 (Medical Grade) Auxiliary Ventricle surfaces became implantable, under identical hemodynamic conditions, for 3-4 weeks with no formation of adherent thrombi. Improvement in blood compatibility is attributed to a preparative technology devised to eliminate various impurities and surface inhomogeneities found by physico-chemical and electron microscopic techniques in Silastics which thrombosed rapidly in uzuo. These imperfections were traceable to processing methods and/or resulted from properties inherent to the uncured elastomer. I n addition to traces of molding cavity exudates, 2,4-dichlorobenzoic acid, a decomposition product of the Silastic's peroxide catalyst, can contaminate finished elastomers in amounts varying according to curing conditions. Sodium stearate solutions used to remove impurities were found to deposit microlayers which became irreversibly adsorbed as a result of transformation into stearic acid insoluble in neutral aqueous media. Aided by the lack of strong bonding between the filler and some of the Silastic's poly-(dimethylsiloxane), certain molding conditions can expose silica particles in submicroscopical domains inducing clots. Demonstrated by electron micrographs and transmission sections, processing technology was found to affect the submicroscopic structure which can alter the compatibility of otherwise identical Silastics within wide ranges. These factors are recognized as important determinants of compatibility.