Immunoelectron microscopy was utilized to detect type V collagen in human amnion. Monospecific antibodies to type V collagen were detected with protein A-gold conjugates in tissue sections and epoxy-embedded sections of human amnion. Type V collagen was localized to the immediate vicinity of the basal lamina, but was distinct from laminin and type IV collagen, which localized only to the lamina lucida and lamina densa, respectively, of the basal lamina. At high magnification, 12 nm unbanded fibrils were seen to be labelled by anti-type V collagen antibody; these fibrils extended from the lamina densa of the basal lamina well into the interstitial matrix. In comparison, only the amorphous matrix of the lamina densa showed labelling with anti-type IV collagen antibodies. Anti-laminin antibodies labelled the lamina lucida. Quantitative analysis of grain distribution revealed the laminin labelling to be centered over the distal half of the lamina lucida (mean distance from the cell surface = 70 nm). In contrast, type IV collagen was centered over the lamina densa (mean = 115 nm). Both distributions were essentially Gaussian and distinct from the broad distribution of type V collagen. Type I collagen fibers with characteristic 67 nm periodicity were unlabelled with antibodies to type V collagen, although labelled type V fibrils were frequently enmeshed among the type I fibers. Antibodies to type I collagen labelled these fibers but not the type V fibrils. The results indicate that in human amnion, type V collagen is a 12 nm diameter, unbanded fibril which extends from the lamina densa of the basal lamina into the adjacent interstitial matrix. We hypothesize that type V collagen functions as a network of anchoring fibrils between the cell basal lamina and the extracellular matrix, especially type I collagen fibres. Type V collagen thus appears to be a unique interstitial collagen.