Methylene blue is strongly and reversibly adsorbed on to acidic groups in the structure outside the osmotic membranes of the human erythrocyte and
Escherichia coli
. This fact has been utilized to determine the numbers of acidic groups on the exterior surfaces of these cells. A necessary feature of the method is the use of electrophoretic measurements to indicate the progress of the titration. A semi-quantitative analysis of the results on the basis of the Stern equation leads to the required number of groups/cm
2
cell surface. Although the results are internally quite self consistent they have an absolute accuracy of only approximately ± 20 %. It is shown that in order to achieve greater absolute accuracy more precise information is required on the porosity of the outer regions of the cell walls. As expected, and in contrast to the surface charge found directly from electrophoretic measurements via the Gouy equation, the number of acidic surface groups is independent of the electrolyte concentration in the surrounding solution. The relative magnitudes of the surface charge originating from surface acidic groups and that charge caused by adsorption or desorption of bulk phase electrolyte have been determined.
