The diffusion coefficients of monodisperse polystyrene latex spheres in solutions of polymerized actin were measured using dynamic light scattering. Four different probes with radii R, ranging from 50 to 500 nm, were separately used in actin solutions with concentrations c, ranging from 1.5 to 21 microM, which had been polymerized with either 1 mM MgCl2, 1 mM CaCl2, or 100 mM KCl. Under all conditions, and at four different scattering angles in the range of 30 degrees-90 degrees, the measured average diffusion coefficients D of the probes were systematically smaller for samples of increased actin concentration or of increased probe radius. Control experiments indicated that the probes did not bind to the actin. These data for Mg2+- and Ca2+-polymerized actin agree and were found to be quite well summarized by the scaling relation D/D0 = exp[-alpha R delta c nu], where D0 is the measured diffusion coefficient of the probes in water (and, as also measured, in the starting actin solutions prior to polymerization with added salt), with values of delta = 0.73 +/- 0.05, nu = 1.08 +/- 0.09, and alpha = (1.1 +/- 0.6) x 10(-3) (with c in microM and R in nm). Data for KCl-polymerized actin show much more restricted diffusivities of the probes at comparable actin concentrations. Inhomogeneities in the solution are reflected in the "effective polydispersity" of the probe diffusion coefficients, which depend on local microviscosity differences.