The properties of a new scintillation detector system for use in dosimetry of high-energy beams in radiotherapy have been measured. The most important properties of these detectors are their hgh spatial resolution and their nearly water-equivalence. Measurements have shown that they have excellent reproducibility and stability, and a linear response versus dose-rate. It is also shown that they have better spatial resolution than ionization chambers and have much less energy or depth dependence in electron fields due to the removal of the influence of the polarization effect. Dose distributions in water, using miniature plastic scintillation detectors, have been measured for different high-energy photon and electron beams.