Combination of heat and various anticancer drugs can exert a synergistic antitumor effect in vitro and in vivo, though the mechanism is not clear. We have previously shown that endogenous tumor necrosis factor (enTNF) acts as an intracellular resistance factor to inhibit the cytotoxic effect of heat by scavenging oxygen-free radicals via the induction of manganous superoxide dismutase (MnSOD). Consequently, we examined whether the suppression of these resistance factors by combining anticancer drugs and heat causes an augmentation of heat-induced cytotoxicity. The human pancreatic carcinoma cell line, PANC-1, constitutively expresses appreciable amounts of enTNF and also demonstrates heat resistance. After treatment of these cells for 15 hr with adriamycin (ADM), the expression of enTNF was decreased by 43%, and MnSOD activity was suppressed by 55%. The cytotoxic effects of the treatment of PANC-1 cells with ADM followed by heat were greater than the sum of those observed with the agents administrated individually. Heat-induced apoptosis was also augmented by pretreatment with ADM. Furthermore, the interleukin-1beta-converting enzyme inhibitor, Ac-YVAD-CMK, reversed the augmented cytotoxicity. Our results indicate that suppression of intracellular resistance factors such as enTNF and MnSOD plays an important role in apoptosis seen after heat and ADM combined therapy.