The room temperature, low field intergrain magnetoresistance (IMR) of the double perovsktite SrFe.sub.0.5MO.sub.0.5O.sub.3 is found to be highly tunable by doping either Ca or Ba into the Sr site. The dopant exerts a chemical pressure, changing the Curie temperature and the magnetic softness. The IMR at optimal doping (Sr.sub.0.2Ba.sub.0.8Fe.sub.0.5Mo.sub.0.5O.sub.3) is approximately 3.5% in 100 Oe, and increases further in high fields. The unprecedented strength of the IMR in this highly spin polarized system provides new grounds for employing novel magnetic materials for new magnetic sensing applications and spin electronics.