We investigated the production of interleukin 6 (IL-6) by a radioresistant human glioblastoma cell line G5 after single radiation events of 3, 6 and 9 Gy. The total cell number and IL-6 concentration in culture supernatant were assessed 24-96 h after irradiation. The radiation impeded or stopped G5 cell growth in a dose-dependent manner, but unexpectedly did not affect the IL-6 concentration in cell culture media that increased in the same range as in non-irradiated cultures. Furthermore, using flow cytometry, we found that the IL-6 positive cells expansion was unaffected by radiation. These findings suggested that this small (about 1%) fraction of G5 cells, constitutively producing IL-6, is highly radioresistant.