Rabbit and thrombastheinic platelet membranes were examined by SDS polyacrylamide gel electrophoresis. Both rabbit and thrombasthenic platelets failed to spread on siliconized glass surfaces and revealed platelet membrane glycoprotein patterns quite different from those of normal human platelet membranes. Typical for normal platelet membranes are four high molecular weight glycoprotein bands. The platelet membranes from rabbits and from one thrombasthenic patient showed only the first major glycopolypeptide with an apparent molecular weight of 135000 and 120000 D respectively. Other platelet membrane glycopeptides (both the carbohydrate and polypeptide moiety) were completely absent in thrombasthenia. The rabbit platelet membrane yet contains two strong polypeptides in this high molecular weight region, however, without corresponding carbohydrate moieties. Therefore, we support the view that the carbohydrate chains from two high molecular weight glycoproteins are of importance for platelet spreading on glass surfaces.