Avascular, undifferentiated mouse kidneys transplanted onto quail chorioallantoic membrane differentiate and become vascularized by quail vessels. The glomeruli which form under these conditions consist of mouse podocytes and quail endothelial cells. Immunohistochemistry has shown that the glomerular basement membrane (GBM) has a dual origin, as integral basement membrane components are produced by both podocytes and endothelial cells. In electron microscopy this GBM is composed of two partially separated layers, an epithelial and an endothelial basal lamina which both have a lamina densa and a lamina rara. These two basal laminas are partially fused, but there are large areas where this fusion does not occur. In some places of incomplete fusion, fibrillar extracellular material is seen between and beneath the GBM. It is concluded that basement membrane components derived from the different species can interact partially, but the fusion is incomplete. The abnormal assembly of the epithelial and the endothelial basal laminas might be due to molecular differences between the components produced by the two cell lineages. In spite of the incomplete fusion, the system used serves as a good model-system to study basement membrane formation, since the cells organize in a histiotypic fashion and form true vascularized glomeruli.