The composition of the extracellular matrix of developing microvessels in plasma clot cultures of rat aorta was studied with light and electron immunohistochemical techniques using affinity-purified antibodies against fibronectin, laminin, and collagen Types I, III, IV, and V. The extracellular matrix of solid endothelial sprouts in young cultures consisted of a delicate fibrillary network of fibronectin and Type V collagen and of patchy amorphous deposits of laminin and Type IV collagen. Rare fibrils of collagen Types I and III were also observed. Fibronectin stained intensely and appeared to be the predominant component of the provisional subendothelial matrix during vascular sprouting. As the cultures aged, laminin and Type IV collagen accumulated in the subendothelial space, forming a continuous feltwork around the newly formed microvessels. Patent microvessels were also surrounded by discontinuous deposits of fibronectin and by increased amounts of collagen Types I, III, and V. Ultrastructural studies revealed positive immunostaining for fibronectin, laminin, and collagen Types IV and V in the endoplasmic reticulum and in putative secretory vesicles, indicating active synthesis and secretion of these molecules by the endothelial cells. These observations indicate that the microvascular extracellular matrix undergoes significant dynamic changes during capillary development. The different composition and structural organization of the extracellular matrix at various stages of angiogenesis may have important effects on endothelial behavior and capillary morphogenesis.