Cleavage of the extracellular matrix (ECM) by proteolysis unmasks cryptic sites and generates novel fragments with biological activities functionally distinct from those of the intact ECM molecule. The laminin G-like (LG)4-5 fragment has been shown to be excised from the laminin alpha 4 chain in various tissues. However, the functional role of this fragment has remained unknown to date. To investigate this, we prepared alpha 4 LG1-3 and alpha 4 LG4-5 fragments by elastase digestion of recombinant alpha 4 LG1-5, and examined their effects on de novo adipogenesis in mice at the site of injection of basement membrane extract (Matrigel) and fibroblast growth factor (FGF)-2. Although the addition of whole alpha 4 LG1-5 suppressed adipogenesis to some extent, the alpha 4 LG4-5 fragment could strongly suppress adipogenesis at a concentration of less than 20 nM. Addition of the alpha 4 LG4 module, which contains a heparin-binding region, had a suppressive effect, but this was lost in mutants with reduced heparin-binding activity. In addition, antibodies against the extracellular domain of syndecan-2 and -4, which are known receptors for the alpha 4 LG4 module, suppressed adipogenesis. Thus, these results suggest that the cryptic alpha 4 LG4-5 fragment derived from the laminin alpha 4 chain inhibits de novo adipogenesis by modulating the effect of FGF-2 through syndecans.