Large negative bulk magnetoresistance has recently been observed in ruthenocuprate materials RuSr2R2-xCexCu2O10-d (up to -47 % at 5 K in a 9 T field). This material exhibits -MR at a hole doping range (in the CuO2 plane) of p = 0 - 0.06 and is superconducting at higher hole concentration. We have recently observed large bulk positive magnetoresistance in the underdoped phase of PrBa2Cu3O7-d (p = 0.01; MR9T(4 K) = 70%). The MR decreases as the hole concentration in the CuO2 plane increases and superconductivity is approached. We aim to determine the mechanism of the positive MR in PrBa2Cu3O7-d by monitoring the change in magnetic structure with field at low temperature.