The Arrhenius plot of the rate of V79 Chinese hamster cell inactivation due to hypothermia has a “break” around 7–10°C with optimum storage temperature for unprotected cells being about 10°C. Addition of the membrane lipid perturber, butylated hydroxytoluene, improves survival of cells when compared to controls at temperatures below this break but not above. Arrhenius plots of growth rates of the cells show breaks at 30 and 40°C. Measurements of membrane fluidity by electron spin resonance or membrane polarization anisotropy by fluorescence spectrophotometry techniques as a function of temperature in these cells also reveal “breaks” centered around 8 and 30°C. Hence, the changes in the rate of cell inactivation and growth as a function of temperature may be related to membrane lipid phase changes.