Mutations in LAMA2 cause severe congenital muscular dystrophy accompanied by nervous system defects [1]. Mice homozygous for the dy^2J allele of LAMA2 express a laminin α2 subunit that has a deletion in the amino-terminal domain VI, providing an animal model for study of the molecular basis of congenital muscular dystrophy [2,3]. Domain VI is predicted to be involved in laminin polymerization, along with amino-terminal domains from laminin β and γ chains [4]. In a solution-polymerization assay, we found that purified dy^2J laminin assembled poorly and formed little polymer, in contrast to wild-type muscle laminin. Furthermore, dissolution of the collagen IV network caused dy^2J laminin to be released into solution, indicating that laminin polymers within the skeletal muscle basement membrane were defective. In addition to loss of polymerization, dy^2J laminin had a reduced affinity for heparin. Finally, recombinant laminin engineered with the dy^2J deletion was more sensitive to proteolysis and was readily cleaved near the junction of domains V and VI. Thus, the dy^2J deletion selectively disrupts polymer formation, reduces affinity for heparin, and destabilizes domain VI. These are the first specific functional defects to be identified in a muscular dystrophy laminin, and it is likely that these defects contribute to the abnormalities seen in dy^2J/dy^2J muscle and nerve.