A discussion of a microelectrophoretic mobility apparatus (Zeta-Meter) is presented in connection with some prerequisite studies with a model dispersed system. The limitations of the fundamental Helmholtz-Smoluchowski formula for calculating zeta potential as it applies to a study of real pharmaceutical systems is presented. A description of the apparatus, including a discussion of the advantages of the darkfield stereoscopic microscope, molybdenum-platinum electrode system, cylindrical, tube-type, plastic electrophoresis cell, and the automatic sample-transfer device is also presented. The usefulness of zeta potential in developing pharmaceutically stable, flocculated suspensions is illustrated with three different examples: ( a ) an adrenocorticoid in which flocculation was directly achieved by varying the concentration of electrolyte added; ( b ) a system in which flocculation was initially induced in the absence of an electrolyte; ( c ) the developmenr of a diagnostic system for blood-cell agglutination testing based upon the principles of zeta potential.