The dielectrophoretic force experienced by a range of cell types has been examined with the aim of investigating possible applications of dielectrophoresis to Biotechnology. Measurements of the dielectrophoretic response of cells used a new optical measurement system developed to provide a rapid, simple and accurate method for studying the dielectrophoretic behaviour of micro-organisms over the frequency range lHz to 4MHz. 7be cell types used in these experiments were bacteria, mammalian cells and a range of Friend Murine Erythroleukaernia cell lines. It has been found that the dielectrophoretic response of a particle may be described in three distinct regions. At frequencies below 20OHz the dielectrophoretic force is found to be greatly influenced by the presence of a net surface charge on the cells, causing a large, hitherto unobserved4 collection at low frequencies. Over the mid-frequency range (20OHz to lOkHz) the collection is found to be controlled by the effective conductivity of the cells whereas above lOkHz the dielectrophoretic response is dictated by the dielectric permittivity of the cells. Investigations into die effects of suspending medium conductivity on the dielectrophoretic response showed that for frequencies below IMHz the magnitude of the force reduced with increasing media conductivity and became directed away from the regions of high field intensity when the media conductivity was greater than that of the cells. Experiments using mammalian cell lines revealed that on the chemically-induced transformation of a cell line from a cancerous state to a near-normal state, an increase in the effective surface conductivity of the cell occurs. This is thought to be related to an increase in membrane rigidity, associated with the increased membrane content of fatty acid molecules such as cholesterol and protein cross-linking effects. A similar effect was also seen in HeLa cells exposed to formaldehyde. The implications of these results to the biotechnological applications of dielectrophoresis are also discussed.