Laminin is a basement membrane-specific glycoprotein (800 kDa) consisting of three chains: A, B1, and B2. Laminin has diverse biological functions, which include stimulating epithelial cell growth and differentiation. We have isolated two overlapping cDNA clones that span 5.9 kilobases and code for the entire B1 chain of mouse laminin. The nucleotide sequence of the clones reveals a 5358-base pair open reading frame that potentially codes for 1786 amino acids, including 20 amino acids of a presumptive signal peptide. Analysis of the deduced protein sequence predicts that the B1 chain has seven distinct domains that include cysteine-rich repeats, alpha-helical, and globular structures. Part of the cysteine-rich region is homologous to epidermal growth factor and other proteins that contain epidermal growth factor-like repeats.