The expression of class II major histocompatibility (MHC) antigens is central to the mounting of a successful immune response. Understanding the molecular mechanisms involved in the induction of class II MHC expression may therefore provide the knowledge necessary to manipulate the immune system appropriately. We are particularly interested in the induction of class II MHC antigens on cells in the brain, because of the potential involvement of such brain cells in the initiation and perpetuation of autoimmune-like diseases of the central nervous system. We examined the mechanisms involved in interferon-gamma (IFN-gamma) induction of class II MHC antigens on a human glioblastoma multiforme cell line. This paper describes the identification of a 297-bp (base pair) fragment of the class II MHC DR alpha chain gene which is involved in IFN-gamma induction. We were able to identify this IFN-gamma responsive sequence by preparing recombinant plasmids containing 5' flanking pieces of the human DR alpha chain gene placed upstream of the indicator gene, chloramphenicol acetyltransferase (CAT). These recombinant plasmids were transfected into human glioma cells which were then cultured in the presence or absence of IFN-gamma. After 48 hr, transient expression of CAT was assayed by thin layer chromatography. CAT enzyme activity was significantly increased only in IFN-gamma-treated cells. This increase was also reflected at the RNA level in that IFN-gamma treatment resulted in higher CAT transcripts. A computer homology search revealed a possible consensus sequence shared among different IFN-gamma-inducible genes.