-Amyloid protein (A) is a major component of neuritic plaques and cerebrovascular amyloid deposits in the brains of patients with Alzheimer's disease (AD). Inhibitors of A fibrillogenesis are currently sought as potential future therapeutics for AD and related disorders. In the present study, the basement membrane protein laminin was found to bind A 1-40 with a single dissociation constant, K d ϭ 2.7 ϫ 10 -9 M, and serve as a potent inhibitor of A fibril formation. 25 M of A 1-40 was incubated at 37°C for 1 week in the presence of 100 nM of laminin or other basement membrane components, including perlecan, type IV collagen, and fibronectin to determine their effects on A fibril formation as evaluated by thioflavin T fluorometry. Of all the basement membrane components tested, laminin demonstrated the greatest inhibitory effect on A-amyloid fibril formation, causing a ninefold inhibition at 1 and 3 days and a 21-fold inhibition at 1 week. The inhibitory effects of laminin on A fibrillogenesis occurred in a dosedependent manner and were still effective at lower concentrations. The inhibitory effects of laminin on A 1-40 fibril formation was confirmed by negative stain electron microscopy, whereby laminin caused an almost complete inhibition of A fibril formation and assembly by 3 days, resulting in the appearance of primarily amorphous nonfibrillar material. Laminin also caused partial disassembly of preformed A-amyloid fibrils following 4 days of coincubation. Laminin was not effective as an inhibitor of islet amyloid polypeptide fibril formation, suggesting that laminin's amyloid inhibitory effects were A-specific. To identify a potential A-binding site(s) on laminin, laminin was first digested with V8, trypsin, or elastase. An A-binding elastase digestion product of ϳ120 -130 kDa was found. In addition, a ϳ55 kDa fragment derived from V8 and elastase-digested laminin interacted with biotinylated A 1-40. Amino acid sequencing of the ϳ55 kDa fragment identified a conformationally dependent Abinding site within laminin localized to the globular repeats on the laminin A chain. These studies demonstrate that laminin not only binds A with relatively high affinity but is a potent inhibitor of A-amyloid fibril formation. In addition, further identification of an A-binding domain within the globular repeats on the laminin A chain may lead to the design of new therapeutics for the inhibition of A fibrillogenesis.