The magnetoresistance of mechanically induced grain boundaries in La0.7Ca0.3MnO3 thin films is investigated. The grain boundaries are fabricated by a mechanical deformation of the LaAlO3 substrate prior to film deposition. During film deposition, the deformed substrate region induces some growth disorder in the film leading to a wide grain boundary. The resulting structures show a reproducible, substantial magnetoresistance in magnetic fields below 2 kG. We discuss the temperature and direction dependence of the magnetoresistance and argue that two contributions due to spin-polarized electron tunneling and the suppression of magnetic frustration by a magnetic field can be identified. © 1998 American Institute of Physics.