WE STUDIED THE effect that treating two types of glioblastoma cell lines, U-87 MG and U-251 MG, with interferon (IFN)-γ had on their susceptibility to lysis by lymphokine-activated killer (LAK) cells. We also examined the participation of cell-adhesion molecules and major histocompatibility complex (MHC) class I and II antigens present on the target cells in lysis by LAK cells. Treatment with IFN-γ (1000 U/ml) for 48 hours resulted in the increased expression of both intercellular-adhesion molecule 1 and MHC class I antigens on tumor cells. In addition, untreated tumor cells expressed neural-cell-adhesion molecules and MHC class II antigens highly, but their expression was not affected by IFN-γ treatment. These changes in expression were accompanied by a decreased susceptibility to lysis by LAK cells. Treatment with antisense-intercellular-adhesion molecule-1 oligonucleotide further inhibited LAK lysis of target cells, following treatment with IFN-γ. In contrast, acid treatment of tumor cells after treatment with IFN-γ increased their susceptibility to lysis by LAK cells. These findings suggest that treatment of glioblastoma cells with IFN-γ decreased their susceptibility to lysis by LAK cells, and that this decrease in susceptibility is attributable principally to the increased expression of MHC class I antigen on target cells.