B16 melanoma cells were inoculated s.c. on the hind leg of C57BL mice. The tumor bearing mice were applied to the test of metastasis on the 16-22 days after the inoculation. Experimental procedures of 6 groups were as follows; (1) Untreated tumors were excised. (2) The tumors were heated together with legs in water bath at 44°C for 30 min under anesthesia and the tumors were excised immediately after the heating. (3) Heat treatment was the same to the group (2), then the tumors were excised on day 3 after the heating. (4) The same heating was applied once a day for 3 days, then the tumors were excised on the next day of the last heating. (5) A combined treatment with the heat at 44°C for 30 min and with a single dose of 19Gy X-rays immediately after the heating was given to the tumors, then excised day 3 after the treatment. (6) Another combined treatment with the same heating and with the same X-irradiation a day before the heating was given to the tumors, then excised on day 3 after the heating. The incidences of the metastasis in those mice were pursued for 90 days after the inoculation. When tumors were larger than 200mm³, each treated group indicated a slight increase in the incidence of metastasis, but not markedly. When tumors were smaller than 200 mm³, the incidences of those groups (1) - (6) were 6.0% (6/100), 4.2% (1/24), 16.1% (10/58), 28.6% (6/21), 24.6% (15/61) and 19.4% (6/31), respectively. The incidence of metastasis by the heating at 44°C for 30 min was almost the same level to the incidence by a single dose of 19Gy X-rays. In the small tumor, the incidences of metastasis by the combined and 3 fractionated heating were higher than those by X-irradiation alone or heating alone.