Ultrafiltration membranes capable of retaining macromolecules, but permeable to micro solutes, have been used in a fixed volume diafiltration assembly. The system permits extremely rapid micro solute exchange in protein solutions and is also applicable to the study of binding behavior. Somewhat analagous to equilibrium dialysis, this method permits change of the ionic or micro solute milieu in a fraction of the time formerly required. The effects of variable volume, salt, and protein content on the diafiltration were investigated and predictive equations derived. Limited binding profiles were established for the interaction of methyl orange and calcium with human serum albumin. © 1968.