The effect of low magnetic fields on the transport properties of a manganite compound with magnetic phase separation is studied. The different behavior obtained in samples of La_0.5Ca_0.5MnO₃ related to the way in which the low field is applied is consistent with a picture of changes in the metallic fraction induced by the magnetic field in a field-cooled cycle. Using a simple model of conduction through a binary mixture, the interrelation between magnetoresistance, metallic fraction and percolation temperature is accounted for. A new physical coefficient relating magnetic field and metallic fraction emerges as the relevant parameter in the description of phase separated manganites.