C3H/He mice were inoculated i.p. with syngeneic MM2 tumor cells (2 X 10(6) cells/mouse), and they were subsequently treated by systemic hyperthermia with microwave irradiation (2450 MHz) alone, administration of 5-FU alone, or a combination of the both. The mice were exposed to hyperthermia for 5 minutes everyday with an output of 10, 20 or 30 watts. 5-FU was administered i.p. for 3 days at dose levels of 2.5 mg/kg, 5 mg/kg or 10 mg/kg body weight. The mean survival time and the mean relative body weight were recorded. From these data, tumor-suppressing effects of the treatments were analyzed by one-way variance analysis or two-way variance analysis. The mice treated with microwave irradiation alone or 5-FU alone survived significantly longer than the control mice receiving no treatment (P less than 0.01). However, no significant difference in mean survival time was observed between the microwave group and the 5-FU group. The mean relative body weight was similar among the microwave, the 5-FU and the control group. When the mice were treated with microwave irradiation and 5-FU, significant difference was observed in the mean survival time (P less than 0.01), and the combination of microwave irradiation with an output of 20 W and 10 mg/kg of 5-FU seemed to be optimal. Determination of an optimal combination was difficult with the mean relative body weight, because body weight decreased due to side effect of the combination therapy.