The effects of prostaglandin D2 (PG D2) and X-ray radiation on the growth and cell survival of C-6 rat glioma cells were evaluated. Exponentially growing monolayer cultures of C-6 glioma cells were incubated with various concentrations of PG D2 for 24 hours prior to X-ray irradiation. PG D2 exposure was associated with an inhibition of cell growth and a reduction in cell survival (IC50 = 5.2 micrograms/ml). The combination of PG D2 and X-ray radiation significantly increased cell killing in comparison with either PG D2 or X-alone. The enhancing factor measured at a survival level of 10% was determined. PG D2 potentiation of radiation toxicity could be related to a blockage of a G1 phase of the cell cycle.