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In order to study the influence of far infrared rays on bacteria, the number remaining of bacteria radiated with a far infrared radiator was compared with that heated in a water bath. The number of the bacteria was determined by the method of reduction cultiviation and the maintenance of temperature close to 60℃ for 30 minutes was taken as standard for inactiviation of asporogenic bacteria by thermal treatment. The temperature of solutions, in which the number of bacteria was about 2.3×10^7/ml at first, was raised severally from room temperature to about 56℃ in both sterilizing methods. The results were as follows : 1. In order to keep the solution temperature in the neighborhood of 56℃ over 10 minutes by slow raising of temperature with the above condition duly considered, it was most suitable to impress 90V continuously on the radiator at the radiating distance of 20 cm. 2. After treatment of 25 minutes from room temperature to about 55℃, while the numbers of E. coli bacteria decreased to less than 2×10/ml in both cases, the number of S. aureus bacteria reduced to 2.8×10^2/ml in the radiator box and 3.2×10^4/ml in the water bath, showing a big difference between the two sterilizing methods. 3. After treatment of 25 minutes from room temperature to about 70℃ in the water bath, the number of E. coli was reduced to less than 2×10/ml and S. aureus to 2×10^3/ml. In this experiment, the solution temperature rose in the radiator box more slowly than in the water bath. Consequently, the radiation up to about 55℃ was found to be more effective than heating in the water bath up to about 70℃ in sterilizing of some species of bacteria, such as S. aureus.