The dependence of the light yield and the transmission on magnetic field has been measured with different methods up to 0.45 T for the plastic scintillators NE-102A, SCSN-38, and Polivar. The scintillators were excited by 25 MeV protons, 5.9 keV X-rays and UV light. When excited with ionizing radiation an increase of light yield is observed. For SCSN-38 of 2.6 mm thickness it amounts to 0.3%, 0.9%, 1.1% and 3.3% at 1, 10, 100 and 450 mT, respectively. NE-102A behaves similar, whereas an acrylic scintillator shows a stronger field dependence. The effect is independent of the direction of the field but increases for the Polivar scintillator with its thickness. No change in the decay time of the scintillator has been observed. The response of the scintillators did not change when excited by UV light in a magnetic field. (orig.).