The effect of hyperthermia and/or nicotinamide (200 mg/kg of body weight) on the tumour growth delay induced by radiation was evaluated in a C3H mouse mammary adenocarcinoma. The study showed a radiosensitizing effect of hyperthermia and of nicotinamide but the combination of all three modalitites showed no increased tumor growth delay compared to hyperthermia and radiation alone. The tumour growth delay induced by hyperthermia was not modified by nicotinamide.