A four-point probe, suitable forin situsurface resistance and magnetoresistance (MR) measurements of ultrathin films, is described. The four-point probe head is mounted and fixed at the center of an electromagnet with the probes perpendicular to the applied magnetic field direction. The ultrathin magnetic film sample can be rotated to face the probe and moved to make contact with the probe using the sample manipulator. The resistance and MR of the magnetic ultrathin film can be measured. At the same sample position, the hysteresis loops can be measured using the surface magneto-optical Kerr effect technique. ©1998 American Institute of Physics.