We report on the experimental observation of large magnetoresistance in polyfluorene organic light-emitting diodes (OLEDs). Very similar magnetic field effects (MFEs) of comparable magnitude were also observed in electroluminescence and photocurrent measurements. We provide a comprehensive overview of these three types of MFE. To the best of our knowledge, the mechanism causing these MFE is currently not known. Moreover, we show that these experiments do not allow determination whether the MFE acts on the carrier density or carrier mobility making any attempt of explaining it ambiguous. As a remedy, we performed magnetoresistance measurements in holeonly OLEDs and show that the MFE acts on the carrier mobility rather than carrier density.