Laminin is a noncollagenour glycoprotein isolated from a transplantable mouse tumor producting basement membrane (BM). Purified antibodies to laminin do not cross-react with other known BM antigens including type IV collagen, fibronectin, bullous pemphigoid antigen, and a BM proteoglycan. Using immunofluorescence, laminin is localized in the BM zones of those human, chick, guinea pig, bovine, monkey, rat, and mouse tissues examined. Epithelial and endothelial cells in culture synthesize laminin while mesenchymal cells do not. By immunoelectron microscopy, laminin was localized to the lamina lucida of human epidermal BM and of mouse esophagus epithelial BM. The wide distribution of laminin among diverse tissues and species, and in early stages of embryonic development suggests that laminin is an ubiquitous component of basement membranes.