Separations processes in biotechnology are defined by the nature of the product and its application. High degrees of purity which approach or attain molecular homogeneity may be required for some products, whereas simply the absence of conflicting activity is tolerated for others. Obtaining intracellular compounds as pure products requires a process comprising harvesting, product release, clarification, e~ richment and fractionation. The early stages of such processes are characterized by attempts to maximize the product yield at the expense of retaining major contaminants. Usually, only then are high-resolution steps applied. Interest in exploiting aqueous twophase systems (Fig. 1) stems from their ability to combine several features of the early processing steps in only one or two partitioning operations. Thus, the technique has been devdoped as a primary purification step in which overall recovery, together with the removal of insolubles and major classes of contaminant, have been paramount. Further fractionation has been confined to conventional high-resolution techniques t. The advantages of the technique 1, 2 include:• substituting difficult solid-liquid sepa~ tions;• linearity of scale-up from the laboratory bench over several orders of magnitude;• rapidity using continuous mixers and centrifugal separators.