The author investigated the effect of frequent thermal bath on blood catalase in healthy persons, liver injured patients and rabbits. The blood catalase level of rabbits tends to increrease after a radioative thermal bath at a temperature of 42°-43℃ in " YAMADA-KU KYODO-YU " . but it tends to decrease after a thermal bath in the Labolatory, which Radon content is about one tenth of the former, namely 40-70×10(-10) Curie units per liter. By the repeated thermal baths of daily three times the blood catalase level of rabbit tends to fall lower after each bath. In healthy persons no remarkable change was shown in the initial blood catalase level by the frequent thermal baths of daily three times at 42°-43℃for 5 minutes. But in liver injured patients a gradual fall of initial level and a slow recovery were seen after each thermal bath.