In order to study the effect of radioactive thermal bath on mineral substances in blood, the author bathed rabbits in Misasa Hot Spring (Yamada-Yu) once daily for 5 minutes dnring the successive 3 weeks, and determined potassium, calcium, sodium, chloride and protein levels
in serum before and after the bath every week. The radon content of Yamada-Yu was 300-600×10(-10) curies per liter, its water temperature being 42-44°C. at that time. In the early stage of serial baths calcium decreased, and potassium, sodium, chloride and protein content increased. But in the later stage a reversed tendency was recognized. It was concluded that radioactive thermal baths of Misasa had no peculiar effect on mineral substances in rabbit's serum, compared with the effects of the other kinds of thermal baths