A study is made of the influence on hemagglutination of various physicochemical parameters, such as: cell shape, cell distance, extracellular colloid-osmotic pressure, degree of hydration of the cell surface, and cell ζ-potential. To a varying extent these are changed by: the addition of divers soluble polymers, the action of enzymes, the presence of various salt ions, centrifugation, agglomeration at low ionic strength, complex coacervation with cationic polymers, and the interaction with different kinds of antibodies. Among the more important parameters are cell distance and cell shape (particularly when the latter tends to spiculation), while the influence of ζ-potential appears to be of fairly minor consequence. The action of dissolved polymers is mediated through polymer bridging, increase in extracellular colloidosmotic pressure, decrease of cell surface hydration, and change in cell shape. A peculiarity of dextran is that in addition to all of the above, it causes pronounced erythrocytic spiculation.