Dielectrophoresis (DEP) has been widely studied for its potential as a biomarker-free method of sorting and characterizing cells based upon their dielectric properties. Most studies have employed voltage signals from similar to 1 kHz to no higher than similar to 30 MHz. Within this range a transition from negative to positive DEP can be observed at the cross-over frequency f(x01). The value of f(x01) is determined by the conductivity of the suspending medium, as well as the size and shape of the cell and the dielectric properties (capacitance, conductivity) of its plasma membrane. In this work DEP measurements were performed up to 400 MHz, where the transition from positive to negative DEP can be observed at a higher cross-over frequency f(x02). SP2/O murine myeloma cells were suspended in buffer media of different osmolarities and measurements taken of cell volume, f(x01) and f(x02). Potassium-binding benzofuran isophthalate (PBFI), a potassium-sensitive fluorophore, and flow cytometry was employed to monitor relative changes in intracellular potassium concentration. In agreement with theory, it was found that f(x02) is independent of the cell parameters that control f(x01) and is predominantly determined by intracellular conductivity. In particular, the value of f(x02) is highly correlated to that of the intracellular potassium concentration.