Colloidal gas aphrons are microbubbles 10-100 mu m in diameter created by intense stirring of a surfactant solution. They are composed of a gaseous inner core surrounded by a surfactant film. They have a high surface area, are relatively stable and have the ability to interact with proteins through electrostatic interactions. These characteristics make colloidal gas aphrons ideally suited for use as a downstream processing technique. In this study the effect of different process parameters on the recovery of three proteins (conalbumin, ribonuclease A and ol-chymotrypsinogen A) was studied for a protein-ACT (sodium bis-(2 ethylhexyl) sulfosuccinate) system. High enrichment ratios (up to 8) and high recovery of the protein (85%) are found under optimum conditions.