The intermolecular interaction force of actin was studied by a dynamic light scattering technique. The Mutual diffusion coefficients (D) of monomeric actin were accurately determined in a G-buffer with a low concentration of KCl from 0 to 10 mM. The translational diffusion coefficient was obtained as D-0 = (87 +/- 3) x 10(-12) m(2)center dot s(-1) at 25 degrees C and pH 7.4, which gives a hydrodynamic radius of monomeric actin of r(H) = 2.8 +/- 0.1 nm. The Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, assuming electrostatic and van der Waals potentials, failed to describe the change in interaction parameter (gimel) with KCl concentration, but the extended DLVO theory succeeded if an additional repulsive potential was assumed. The Hamaker constant of actin in the Ca2+-ATP bound state was determined for the first time as A(H) = 10.4 +/- 0.6 k(B)T.