A key physical event during the cooling of cells is the temperature at which the intracellular compartment undergoes vitrification. Recently (http://dx. doi.org/10.1371/journal.pone.0066207) we have measured the intracellular glass transition temperature (Tg) of bacteria using differential scanning calorimetry (DSC). Here we report the first data on the intracellular Tg of a mammalian cell cooled with and without a cryoprotectant (Me2SO). Jurkat cells (an immortalized T cell line) were concentrated and a pellet of approximately 30 mg cooled in a DSC, SnowMax was added to induce ice nucleation. On warming an intracellular glass transition was detected by the deviation in the first derivative of the heat flow, associated with a change in heat capacity as the cells devitrify at Tg. The transition temperature measured by DSC is Tg', that is the Tg of the maximally freezeconcentrated intracellular matrix. In the presence of Me2SO intracellular Tg was measured to be - 40 C, in controls (no Me2SO) Tg occurred at higher temperatures. In further experiments the temperature of the lipid phase transitions were analyzed by FTIR, and were measured to occur above freezing temperatures. The implications of the measured values of Tg’ for the basic cryobiology of mammalian cells will be discussed.