The diffusion coefficient and hydrodynamic radius of laminin-1 (LN-1) in aqueous solutions were investigated by using a dynamic light scattering (DLS) technique. The autocorrelation functions could be analyzed assuming two relaxation modes, fast and slow, with exponential decay. We primarily focused on the fast mode, because the mass of it represented 99% of the total mass. The unit corresponding to the fast mode had a hydrodynamic radius of 20.6 +/- 0.5 nm in both tris- and MOPS-buffered solutions of 150 mM NaCl and pH 7.2 at 25 degreesC. Intermolecular interactions between these essential units were attractive, and the Hamaker constant in tris-buffered solution was estimated as 5.2 k(B)T according to DLVO theory. When the magnesium was added to the tris-buffered solutions, the attractive force strongly increased with increasing magnesium concentration. This change was not explained by the charge-screening effect of magnesium ions in the framework of the classical DLVO model.