The electrostatic properties of lens fiber cell membranes have been investigated by recording the electrophoretic mobility of membrane vesicles formed from isolated fiber cells. The vesicles appear to be sealed and have external surfaces that are representative of the extracellular surface of fiber cells. The average mobility of a vesicle in normal Ringer's solution was 0.9 microns/s per v/cm, which gives a zeta potential of -9 mV, a value similar to that reported for other cells (McLaughlin, S. 1989. Annu. Rev. Biophys. Biophys. Chem. 18:113-136.). There was no significant difference in the mobility of vesicles formed from peripheral, middle cortical, or nuclear fiber cells. Vesicle surface changes were titrated using Ca and Mg and each had a pK of approximately 2, which is similar to that for the most common phospholipids. We also titrated these charges with varying pH and found the most significant changes in mobility at pH values between 5 and 6. The majority of lipids found in biological membranes are not titratable in this pH range, so the pH effect is probably through a membrane protein charged group. These experimental data in conjunction with the previously measured extracellular voltage gradient (Mathias, R. T., and J. L. Rae. 1985. Am. J. Physiol. 249:C181-C190) imply that electroosmosis can generate a fluid velocity of approximately 0.6 mm/h, directed from the aqueous or vitreous toward the center of the lens, along intercellular clefts.