Laminins are large hetero-trimeric, non-collagenous glycoproteins composed of alpha, beta, and gamma chains. This MAb reacts with laminin B2/1 chain of ~210kDa and does not cross-react with other basement membrane components or fibronectin. Its specificity was established by immunoprecipitation and immunofluorescence of human skeletal muscle and kidney with laminin chain-specific MAbs. Epithelial sheets in vivo are separated from the mesenchymal elements of the stroma by a thin layer of a specialized type of extracellular matrix termed the basement membrane (BM). This structure consists of individual components, some of which are ubiquitous in BMs and some are not. The ubiquitous ones comprise laminin (LN), entactin/nidogen (EN), collagen type IV (CIV), and large heparan sulfate proteoglycan (HSPG), which interact specifically with each other to form a continuous and regular BM. Alterations of BM integrity, from local discontinuities up to complete loss, are described in many types of human and animal epithelial neoplasms. This MAb stains uniformly all human and murine basement membranes.