The 2000 series of internally stirred ultrafiltration cells are designed for rapid concentration or purification of macromolecular solutions in volumes up to 2000 mL. All models feature stainless steel construction and include a built-in, replaceable stirring motor con-trolled by a separate solid state power supply. Speed settings are continuously variable from 0 to 340 RPM (± 9%). The motor is magnetically coupled to a stirring bar which maintains fluid movement during operation, thereby reducing the negative effects of concentration polarization (i.e., the buildup of concentrated solutes on the membrane). Typical flux for the 2000 series of stirred cells ranges from 600 to 800 mL/hr.