The feasibility and scalability of extraction in detergent-based aqueous two-phase systems for the separation of proteins from culture broth is demonstrated. At the same time the large-scale production of a fusion protein and the influence of cultivation scale on the efficiency of separation were investigated. An amphiphilic fusion protein EGIcore-HFBI was chosen, consisting of the catalytic core of the cellulase endoglucanase I and the small protein hydrophobin I, expressed homologously in Trichoderma reesei. Using the technical nonionic detergent Agrimul NRE 1205 the separation was successfully scaled up to 1200 l. No differences in yield or in partition coefficient were observed at 10 ml and 1200 l scale. Changes in the fermentation temperature and scale, however, can influence the properties of the protein and thus alter partition coefficient and yield. The decreased separation efficiency appears to be correlated with changes in glycosylation at lower cultivation temperatures. (C) 2003 Published by Elsevier Ltd.