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 alpha2 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 beta and gamma 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.