Purpose: Investigation of the in vitro cytotoxic effect of X-rays, either alone or combined with cisplatin on early passage cell cultures derived from human glioblastoma multiforme biopsy tissue. Materials and methods: Fresh tumour specimens from four patients were processed to cell cultures. The U373 glioma cell line was used as a reference. Early passage cell cultures were X-irradiated (0-8 Gy) either alone or in combination with cisplatin (0.5-1 μg/ml). Cell survival was determined by either clonogenic assay or the colorimetric MTT assay. Survival curves were generated and mathematically analysed using the linear quadratic model, to obtain the radiosensitivity parameters α, β, and SF2, i.e., the Surviving Fraction after 2 Gy. Results: Two patient-derived glioma cell cultures and the U373 cell line showed rather high SF2 values of 0.61-0.72 in the clonogenic assay, indicating relative high radiation resistance. Cisplatin alone (1 μg/ml) reduced cell survival by 10-30% (n = 4). When combined with irradiation, a clear additive cytotoxic effect of cisplatin was demonstrated by the unaltered value of the α-parameter for reproductive cell death. Conclusion: Cisplatin exerted an additive rather than radiosensitising cytotoxic effect in uncharacterised patient derived glioma cell cultures.