Purpose : It was demonstrated previously that some radioresistant tumour cell lines respond to decreasing single, low radiation doses by becoming increasingly radiosensitive. This paper reports the response of four radioresistant human glioma cell lines to multiple low-dose radiation exposures given at various intervals. Three of the cell lines (T98G, U87, A7) were proven already to show low-dose hyper-radiosensitivity (HRS) after single low doses; the fourth, U373, does not show HRS after acute doses. Materials and methods : Clonogenic cell-survival measurements were made in vitro using the Dynamic Microscopic Image Processing Scanner (DMIPS) or Cell Sorter (CS) following exposure to 240kVp X-rays one or more times. Results : A consistent, time-dependent hypersensitive response to a second, or subsequent, dose was observed in the cell lines that demonstrated HRS. This time-dependent change in radiosensitivity did not occur in the radioresistant cell line that did not show HRS (U373). In one cell line that demonstrated strong HRS, T98G, a similar time-dependent hypersensitive response was also seen when the cells were irradiated whilst held in the G1-phase of the cell cycle. In this same cell line, significantly increased cell kill was demonstrated when three very low doses (0.4 Gy) were given per day, 4 h apart, for 5 days, compared with the same total dose given as once-daily 1.2Gy fractions. Conclusions : These data demonstrate the possibility that a multipledose per day, low-dose per fraction regimen, termed 'ultrafractionation', could produce increased tumour cell kill in radioresistant tumours compared with the same total dose given as conventional-sized 2 Gy fractions.