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Respiratory syncytial virus in purified suspensions was inactivated exponentially at temperatures from 50° to 0° and at -78°. The rate constants (K) decreased from 11 log. p.f.u./hr at 50° to 0.2 log. p.f.u./hr at 25°. The constants then increased again to 0.7 log. p.f.u./hr at 0°. An Arrhenius plot showed a linear relationship between ln K and 1/T in the range from 50° to 37°, with thermodynamic constants corresponding with those of protein denaturation. As has been reported for other viruses (reviewed by Woese, 1960), a linear relationship was not observed below 37°.
In molar solutions of NaCl, MgSO4, Na2SO4, sodium-phosphate and glucose the virus was almost completely protected against thermal inactivation at all temperatures greater than 0°. The extent of protection decreased with molarity and 0.1 m-solution of these salts failed to protect. At 0° the virus was protected by glucose, but only partially by salts. The protection against thermal inactivation was apparently not an ion effect and the influence of glucose and of molarity suggests an osmotic effect. The unexpectedly high inactivation rate at 0°, as well as the different effects of NaCl and MgSO4, suggest a unique mechanism at this temperature.