Epithelial basement membrane, synthesized by a parietal yolk sac carcinoma in vitro or in the ascites, is free of connective tissue cells and products. Carbohydrate components of this basement membrane were d-galactose, d-glucose, mannose, fucose, deoxyribose, ribose, glucosamine, and sialic acid. Total carbohydrate content was 9.50%. Galactosamine, and uronic acid were absent. This composition is compatible with previously reported analyses of basement membrane from non-neoplastic tissues. There was similarity in four out of five intensity maxima with high-angle x-ray diffraction when comparing basement membrane and collagen. A collagen or collagen-like structure may therefore be a basic structural element in epithelial basement membrane. © 1969.