By means of 20 and 35 Mev proton beams obtained from the University of Minnesota Linac, luminescent effects in Plexiglas Cˇerenkov detectors have been investigated. It was found that 1% of the protons entering the detector produced light pulses which overlapped the mu‐meson Cˇerenkov light distribution. These light pulses correspond to energy losses in a sodium iodide crystal of mainly less than 20 kev. Luminescent effects in the glass face of the particular photomultiplier employed were also studied and found to be quite large.