Laminin was isolated from a rat yolk sac tumor by salt extraction, gel filtration, and affinity chromatography on heparin-Sepharose. The purified laminin gave two polypeptide chains with approximate Mr of 200,000 and 400,000 in sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Its amino acid composition and electron microscopic appearance were similar to those reported earlier for mouse laminin. Carbohydrate analysis revealed 13% carbohydrate consisting of N-acetylglucosamine, galactose, mannose, fucose, sialic acid, and small amounts of N-acetyl galactosamine. The purified rat laminin was immunologically very similar to mouse laminin as recognized by rabbit antibodies but was antigenically distinct when recognized by mouse antibodies.