&NA;The contents of 14 cysts that were located within human intracranial tumors were obtained at surgery by needle aspiration. These tumor cyst fluids (TCFs) were mostly derived from glial tumors (10 cases). TCFs from one metastasis from a mammary carcinoma, one cystic meningioma, one hemangioblastoma, and a cystic acoustic neurinoma were also included. These TCFs were added to primary cultures of human gliomas, established human glioma cell lines, and normal human arachnoid cells in culture. The presence of proliferation‐promoting factors in all cyst fluids could be demonstrated. On the basis of the response patterns of the cultures, it was possible to distinguish different levels of growth autonomy and growth factor sensitivity among these cultures and to speculate about varying degrees of cellular autocrine activation. The TCFs appear to contain factors that are not normally present in fetal calf serum, which is a regular constituent of most cell culture media. Some primary cultured cells as well as cell lines react in an oversensitive manner to the addition of TCFs. (Neurosurgery25:196‐201, 1989)