A gas-discharge probe to monitor the purity of the gas content of a streamer chamber is described. The probe can be placed both in a pipe line and in the walls of the chamber itself. A calibration curve is presented to determine the concentration of air in neon from 0.05 to 100%. The working voltage ranged from 260 to 2250 V. The probe can be used to control gas filling and in other gas-discharge particle detectors.