Human T lymphocytes were stimulated using phorbol myristate acetate and ionomycin. Twenty-four hours post-activation the cells were harvested for DNA content and for measurements using a newly developed cell profiling system employing dielectrophoresis. This system provides individual cell size and dielectrophoresis data for statistically relevant numbers of control and activated cells. From this it was determined that the mean membrane specific capacitance decreased from 13.49 (+/- 4.72) mF/m(2) to 10.62 (+/- 5.13) mF/m(2). This can be related to a 21.3% reduction in the effective membrane surface area associated with membrane topography (e.g. reduction of membrane associated microvilli, blebs and folding), or to other changes of membrane architecture, following cell activation. From cytometric determinations of DNA content, it was concluded that these effects were related to a 3.0-fold decrease of cells in S-phase, and a 1.5-fold increase in G1 cells. This work demonstrates the powerful potential of using dielectrophoresis as a noninvasive tool to follow physiological changes that accompany transmembrane signaling events.