Commercially available latex particles with superparamagnetic inclusions have been characterized by polarized and depolarized dynamic light scattering. The particles are spherical, but the larger ones (R(g) similar to 0.1 mu m) with many magnetic inclusions behave in dynamic light scattering experiments like symmetric tops while the smaller particles do not. The as-received materials had a broad size distribution, but more narrowly distributed particles were prepared by selective filtration for use in probe diffusion and stability studies. The interaction of these negatively charged particles with a linear polyelectrolyte, negatively charged sodium polystyrene sulfonate, was studied by static and dynamic light scattering. Depolarized dynamic light scattering is particularly sensitive to changes in the stability of the colloids or interaction with the polyelectrolyte. The polyelectrolyte can prevent or reverse salt-induced aggregation of the superparamagnetic latex particles. Fluorescence photobleaching recovery experiments to monitor the mobility of fluorescently labeled polyelectrolyte suggest that it is not bound to the latex particles; apparently, a mechanism other than steric stabilization is involved. The dependence of the diffusion of the superparamagnetic latex particles upon added salt is discussed in terms of the ion atmosphere surrounding the polyelectrolyte.