One of the morphologic hallmarks of human gliomas are inflammatory infiltrates with accumulation of macrophages in the tumor site. The signals leading to the macrophage response are only at the beginning of being understood. Novel chemotactic factors that have recently been characterized as secretory products of glioblastoma cells may attract mononuclear cells from the blood. Within the tumor tissue blood-derived monocytes and macrophages of the brain tissue, the microglial cells, may increase in cell numbers due to tumor-derived growth factors. Both astrocytoma cell lines and cultured astrocytes have been shown recently to produce granulocyte- macrophage (GM)-CSF. We show that in vitro not only astrocytoma but also glioblastoma cell lines secrete GM-CSF when stimulated with TNF-α or IL-1. However, there is no evidence for GM-CSF production by glioblastoma cells in vivo: fresh tumor samples lack the mRNA for GM-CSF and the protein is not detectable in the tumor cyst fluids or the cerebrospinal fluids of glioblastoma patients. This contrasts IL-1 and IL-6 that are detectable in the tumor cyst fluids and IL-6 also in the cerebrospinal fluids of the patients. Unlike GM-CSF, transforming growth factor-β2 mRNA is expressed in ex vivo tested glioblastoma tissues. Absence of GM-CSF in vivo may be explained by the presence of tumor-derived inhibitory factors, such as transforming growth factor-β2 and PGE which suppress GM-CSF production by glioblastoma cells in vitro. The accumulation of macrophages at the tumor site may be due to local elaboration of chemoattractants and/or not yet defined growth factors rather than due to GM-CSF production.