S–laminin/laminin β2, a homologue of the widely distributed laminin B1/β1 chain, is a major component of adult renal glomerular basement membrane (GBM). Immature GBM bears p1, which is replaced by β2 as development proceeds. In mutant mice that lack β2, the GBM remains rich in β1, suggesting that a feedback mechanism normally regulates GBM maturation. The β2–deficient GBM is structurally intact and contains normal complements of several collagenous and noncollagenous glycoproteins. However, mutant mice develop massive proteinuria due to failure of the glomerular filtration barrier. These results support the idea that laminin β chains are functionally distinct although they assemble to form similar structures. Laminin β2–deficient mice may provide a model for human congenital or idiopathic nephrotic syndromes.