In the present study, the synergistic effect of mild hyperthermia in combination with gene expression of interferon-beta (IFN-beta) was examined in vitro in the human glioma cell line U87MG. The cells transiently expressed the IFN-beta gene under the control of the mouse mammary tumor virus promoter and were then subjected to temperature elevation (41 degrees C for 1 h). In terms of the cell killing effect, the optimum scheme was obtained by transfection for 4 days before hyperthermia, i.e. rate in the time course of IFN-beta gene expression. The relative specific growth rate decreased to 32% compared with the control while it was only 40% under the hyperthermic conditions. These observations suggest that IFNbeta expression was able to enhance the sensitivity of transfected glioma cells to mild hyperthermia.