Overexpression of oncogenes has been associated with the pathogenesis of some human cancers. The ros oncogene, which encodes a putative receptor with tyrosine kinase activity, has been recently shown to be specifically expressed in high levels in human astrocytoma and glioblastoma cell lines. Using transcription mapping analysis, we surveyed 25 primary astrocytomas of all histological grades, including glioblastomas, and failed to demonstrate elevated expression of ros in these tumors. This difference between the cell lines and the primary tumors may be due to dilution of the ros-positive clones by larger populations of ros-negative cells in the primary tumors or to induction of ros oncogene when the tumors are adapted to tissue culture.