The concept that extracellular matrix materials are involved in the morphogeuetic process is supported by substantial indirect evidence. Essential morphogenetically active materials are obscure with regard to their nature, their mode of action, and whether they are causally involved in tissue interactions. Studies are presented indicating that glycosaminoglycans are components of embryonic epithelial basal laminae, and that materials within the basal lamina which are, at least in part, glycosaminoglycan are required for establishing and maintaining braching epithelial morphogenesis. The tissue of origin and molecular nature of basal laminar glycosaminoglycan are described and speculations are made regarding its possible mode of action in the context of a model for branching morphogenesis.