Recent biochemical and immunohistochemical studies have described several components of basement membranes including heparan sulfate proteoglycan, 2 high molecular weight glycoproteins (fibronectin and laminin), and 2 collagen types (IV and V). These collagens have several properties which distinguish them from other types that are located in the interstitium: (a) type IV forms an amorphous, felt-like matrix, and neither IV nor V is found in large, cross-banded fibrils, (b) both have an increased content of hydrophobic amino acids, (c) the precursor (pro) forms are larger than those of interstitial collagens, (d) type IV contains interruptions within the triple helix, and e) both IV and V are resistant to human skin collagenase but are substrates for selected neutral proteases derived from mast cells, macrophages, and granulocytes. By immunofluorescence staining, type IV collagen has been localized to basement membranes at the dermal-epidermal junction, in capillaries, and beneath endothelial cells in larger vessels. Ultrastructurally it has been shown to be a specific component of the lamina densa. Type V collagen has been localized to the pericellular matrices of several cells types and may be specific for extramembranous structures which are closely associated with basal laminae. Other collagenous proteins have been described which may be associated with the extracellular matrix. One of these is secreted by endothelial cells in culture and by peptide mapping represents a novel collagen type. It is secreted under ascorbate-free conditions and is highly sensitive to proteolytic degradation. It has been proposed that a dynamic reciprocity exists between cells and their extracellular matrix which partially determines cell shape, biosynthesis, migration, and attachment. Examples of phenotypic modulation in several of these phenomena have been shown with endothelial cells grown on different substrates and isolated from different vascular environments.