Interferons (IFNs) are proteins which have antiviral and antiproliferative properties and are known to affect various immunological processes. Some of these activities have been shown to be potentiated by increased temperatures. When cells are subjected to a rise in temperature, the synthesis of the heat-shock proteins (HSPs) is 'switched on.' In this report we demonstrate a synergistic effect of IFN and stress (arsenite treatment or elevated temperature) on the heat-shock response. On the one hand, IFN pretreatment enhances the accumulation of HSP mRNAs and the corresponding protein synthesis after a mild stress and, on the other hand, it amplifies the decrease of the total protein synthesis after a severe stress. Thus in IFN pretreated cells the range of temperatures leading to the heat-shock response is shifted towards common physiological values.