Merosin is a basement membrane-associated protein found in placenta, striated muscle, and peripheral nerve. A 3.6-kilobase merosin cDNA clone was isolated from a placental cDNA expression library. The clone contained a 3.4-kilobase open reading frame, the 3' portion of which includes protein sequences of proteolytic fragments of merosin. The deduced amino acid sequence of the merosin polypeptide was similar to that of the COOH-terminal region of the 400-kDa A chain of laminin. This part of laminin forms the large globule at the end of the long arm of the laminin cross and is thought to contain the neurite-promoting site and the major cell binding site(s) in laminin. The sequence identity between merosin and the laminin A chain in this region is nearly 40%. An antiserum against a synthetic peptide from the middle of the merosin cDNA sequence identified a 300-kDa polypeptide in placental extracts, indicating that the merosin polypeptide is similar in size to the laminin A chain. Intact merosin was isolated from placental extracts and shown to be covalently associated with the laminin B chains and to have a cross-like structure similar to that of laminin. The similarities between merosin and laminin show that both proteins are members of the same family of basement membrane proteins.