Marszalek, P., J. J. Zielinsky, and M. Fikus (1989. Bioelectrochem. Bioenerg. 22:289-298) have described a novel design for measuring the complete dielectrophoretic spectrum of a single cell. From the analysis of the dielectrophoretic spectrum, the membrane conductivity, sigma-membr, and the membrane dielectric permittivity, epsilon-membr, of the cell may be determined according to the theory of dielectrophoresis described by Sauer, F.A. (1985. Interactions between Electromagnetic Field and Cells. A. Chiabrera, C. Nicolini, and H. P. Schwan, editors. Plenum Publishing Corp., New York. 181-202). At F(o), the net force experienced by a single shell sphere in a nonuniform periodic field is zero, and the sphere ceases to move in the field. In other words, at F(o), the effective polarizability, chi-eff, of the sphere (the polarizability of sphere minus the polarizability of the medium) is equal to zero. For biological cells in high conductivity medium, e.g., the isotonic saline, sigma-membr falls below 2 x 10-6 S m-1, where Fo becomes insensitive to sigma-membr, and the method becomes impractical. In a low conductivity medium, 0.3 M sucrose, sigma-membr of cells is generally higher and the method may be applied. Assuming a membrane thickness of 9 nm, epsilon-membr of Neurospora crassa slime cells was determined to be in the range of 8.3-9.4 epsilon-o, and of myeloma Tib9 to be 9.4 epsilon-o, epsilon-o being the dielectric permittivity of the free space. The values for the slime cells were compared with values obtained by the dielectric spectroscopy method which measures average values for cells in suspension.