Previous studies have shown that the antiviral response induced by interferon in murine cells could be degraded after a heat shock. Here we have confirmed that a similar effect occurs also in interferon-treated human HeLa cells subjected to a heat shock. In addition, we have investigated the fate of the interferon-induced, double-stranded RNA-dependent protein kinase in heat-shocked cells. This protein kinase is a Mr 68,000 protein (p68 kinase) which, when autophosphorylated, catalyzes phosphorylation of the protein synthesis eukaryotic initiation factor-2, thus mediating inhibition of protein synthesis. After heat shock of interferon-treated HeLa cells, the double-stranded RNA-dependent autophosphorylation of p68 kinase in cytoplasmic extracts is greatly reduced whereas the phosphorylation of other cellular proteins is not affected. In vivo, autophosphorylation of p68 kinase is also reduced in heat-shocked cells whereas there is no apparent effect on the phosphorylation state of other proteins. In such cells, the interferon-mediated antiviral response becomes modified according to the virus challenge, i.e. these cells remain resistant to vesicular stomatitis virus but become partially sensitive to encephalomyocarditis virus (EMCV) infection. The reduction in the activity of p68 kinase is due to its reduced nonionic detergent solubility occurring during the heat shock period. The resultant reduced detergent extractibility of p68 kinase is dependent on the intensity of the thermal stress. In contrast to the effect after a heat shock, arsenite treatment of interferon-treated HeLa cells induces heat shock proteins, but neither modifies the antiviral response nor affects the extractibility of p68 kinase. These results indicate that the degradation of the anti-EMCV response and reduced p68 kinase activity occur in response to heat treatment independently of the induction of heat shock proteins. The role of p68 kinase in the mechanism of the antiviral response against EMCV and vesicular stomatitis virus is discussed.