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The electrophoretic mobilities and surface areas of tissue culture cells fixed in different morphological states were determined under standard conditions. The apparent surface area of cells fixed in a spread state was found to be approximately two times greater than the surface area of cells fixed in suspension. However, the electrophoretic mobility of spread and non-spread cells was the same indicating that the charge density of the cell surface remains unaltered during these morphological changes. The possible mechanisms for retaining a constant electrophoretic mobility while the apparent surface area changes are discussed and it is concluded that probably the cell surface has undulations which become flattened and thus increase the apparent surface area when the cell spreads out while the charge density remains unaltered.

The electrophoretic mobilities of three different tissue culture cell lines grown in stationary and suspension culture were measured before and after fixation with formaldehyde and osmium tetroxide. Differences or similarities in mobilities of cells from stationary or suspension culture were detectable before and after fixation. However, fixation causes a 10 to 50 per cent increase in mobility and the risks in evaluating cell surface properties on fixed cells are emphasized. The possible mechanisms for the effect of the fixatives on the electrokinetic properties of the cells are briefly discussed in terms of the affinity of the fixatives for charged groups at the cell surface.

The intracellular uptake of neuraminidase by three different cell types was studied by two independent methods: (1) measurement of the electrophoretic mobility of nuclei isolated from cells with or without previous neuraminidase treatment and (2) direct observation by fluorescence microscopy of the uptake of fluorescein iso-thiocyanate labelled neuraminidase. The results indicate that neuraminidase can enter these cells and release intracellular N-acetylneuraminic acid. It is thus possible to account for the observed discrepancy between the amount of sialic acid released from cells and the observed drop in surface charge density.