[en] Using a model system of in vitro human peripheral blood lymphocytes, the effect of low-dose (0.25 to 1.50 Gy) 250-kV_p X ray radiation (1 Gy.min^-1) on the expression of several proto-oncogenes was examined (c-Haras, c-src, c-met, c-jun, c-fos, and c-myc) and β-actin from 0.25 to 17 h post-radiation. RNA was extracted from cells harvested at various times after exposure and examined for levels of particular mRNAs by northern blot hybridisation. A progressive time- and dose-dependent increase in mRNA levels was observed for c-Haras mRNA, while the other proto-oncogenes (c-src, c-met, c-fos, c-jun, and c-myc) examined were variable during the same time period. β-actin levels were initially decreased but at 17 h post-radiation had returned to control levels. A comparison of the rate of c-Haras transcription at 5 and 17 h post-irradiation revealed that c-Haras transcription was higher at 5 h than at 17 h. These findings suggest that the level of specific proto-oncogene expression, particularly c-Haras, may be useful early diagnostic molecular biomarkers for biodosimetry applications. The use of real-time PCR technologies to quantify gene expression changes will also be discussed. (author)