Mobility of positrons in polyethylene.

We have measured the positron drift velocity vs. electric field in polyethylene by observing the Doppler-shift of the annihilation photons. We find that the positron mobility is mu=(10.3+- 1.5)cm(2)V(-1)sec(-1) in agreement with the observations of MacKenzie and Mourino and Brandt, but in disagreement with those of Heinrich.