Abstract:The magnetoresistance (MR) in polycrystalline colossal magnetoresistive compounds follows a behavior different from single crystals below the ferromagnetic transition temperature. This difference is usually attributed to spin polarized tunneling at the grain boundaries of the polycrystalline sample. Here we derive a theoretical expression for the contribution of spin polarized tunneling to the magnetoresistance in ferromagnetic systems under the mean field approximation. We apply this model to our experimental data on the half metallic ferromagnet La 0.7 Sr 0.3 MnO 3 , and find that the theoretical predictions agree quite well with the observed dependence of the spin polarized MR on the spontaneous magnetization.