This study demonstrates interleukin 6 (IL-6) production and release by human glioblastomas. Twenty glioblastoma cell lines were tested for IL-6 bioactivity using an IL-6-dependent cell line (7TD1). All of the lines tested with one exception (LN-229) constitutively released IL-6. A significant induction of IL-6 production and secretion was observed when LN-229 cells were treated with interleukin 1β (IL-1β) or tumor necrosis factor α. Various amounts of IL-6 mRNA were found in five of six cell lines tested. IL-6 mRNA was detected in line LN-229 only when the cells were treated with IL-1β or tumor necrosis factor a, confirming the bioassay data. Glioblastoma cells also produce IL-6 in vivo, (a) IL-6 activity was detected in 11 of 13 cerebrospinal fluids and five of five tumor cyst fluids, (b) IL-6 mRNA was found in four of four tumors, (c) Immunohistochemical analysis showed IL-6 within the tumor cells in 15 of 20 glioblastoma sections. In conclusion, biologically active IL-6 is released by almost all glioblastomas both in vitro and in vivo. The elevated levels of serum acute phase proteins and immune complexes found in glioblastoma patients may be the result of this secretion. © 1990, American Association for Cancer Research. All rights reserved.