Abstract:The response of cell division in the "Page" ( Citrus reticulata Blanco Ï C. grandis Osb. er. Page) protoplasts from embryonic callus to radiation challenge varied with the intensity of the X-ray dose charged. When the protoplasts were irradiated with a dose of 4128 C/kg failure of cell division was completely irreversible, those after 2064 C/kg treatment revealed low frequenee of cell division. Those after treatment with 1161 C/kg of X-ray and kept at - 11 ℃ gave off a 10.2% ~16.9% frequence of division. Some of those viable protoplasm developed into embryoids from which plantlets were regenerated. The LT50 of the leaf protoplasts in 2 of the regenerated plants was markedly lower than that in the controls.