The authors evaluated the effect of liposomal transfection of human gamma-interferon (HuIFN-gamma) gene into human glioma cells and lymphokine-activated killer (LAK) cells, alone and in combination. An HuIFN-gamma gene inserted in a eukaryotic expression vector was entrapped in liposomes bearing positive surface charges. Liposomal gene transfection induced production of HuIFN-gamma and its secretion in culture medium of human glioma cell lines (SK-MG-1 and U-251 MG). At 4 days after transfection, the cells produced 10 to 50 U/ml of HuIFN-gamma in the medium, whereby the major histocompatibility complex (MHC) class I and II antigens, as well as intercellular adhesion molecule-1 (ICAM-1), were induced on the glioma cell surface. The growth-inhibiting effect of transfection-induced HuIFN-gamma was much stronger in comparison with control cultures exposed to 500 U/ml of exogenously added HuIFN-gamma. In addition, 20% to 40% growth inhibition was obtained in the glioma cells when they were treated with LAK cells alone at a 5:1 ratio of effector to target cells. Liposomal transfection of HuIFN-gamma gene into human glioma cells combined with immunotherapy using LAK cells was more effective than either technique alone. The reinforcement of growth inhibition in the case of combined therapy was quenched by anti-ICAM-1 monoclonal antibody, but not by anti-MHC class I or II monoclonal antibodies. These results suggest that the combined effect of liposomal transfection of HuIFN-gamma gene plus LAK cells into human glioma cells is a potentially useful therapy for malignant glioma, and that the mechanisms of the reinforcement of growth inhibition are closely related to the expression of ICAM-1 on the glioma cell surface.