The hyperthermic effect of magnetic particles was examined in an in vivo study of mouse B16 melanoma. Magnetite cationic liposomes (MCLs) have a positive surface charge and generate heat under an alternating magnetic field (AMF) by hysteresis loss. MCLs were injected into melanoma nodules, which were then subjected to an AMF. The mice were divided into four groups: group I (control), group II (hyperthermia at 43 degrees C for 30 min, once), group III (hyperthermia at 46 degrees C for 30 min, once) and group IV (hyperthermia at 46 degrees C for 30 min, twice). Complete tumour regression was observed in 90% of the mice in group IV, while no mice in groups I and II and only 40% in group III showed regression. To examine whether hyperthermia caused immune induction in B16 melanoma, in vitro cytotoxicity assays and rechallenge experiments were performed. Cytotoxic activity was observed in the spleen cells of the cured mice in group IV. In the rechallenge experiment, 66% of the cured mice rejected melanoma cells. These results suggest that hyperthermia using MCLs is an effective therapy for melanoma, since this treatment can kill the tumour cells not only by heat but also by inducing an immune response.