Lysozyme was extracted into an organic reverse micellar phase formed in isooctane with dioctyldimethyl ammonium chloride (DODMAC) and decanol as cosurfactant. The extraction efficiency was measured for different initial lysozyme concentrations, different cations in the aqueous phase, two surfactant concentrations in the organic phase, and various volume ratios between aqueous and organic phases. The presence of different cations, Na+, K+ and Ca2+, in the aqueous phase did not affect the water uptake, but it strongly affected the extraction efficiency. Increasing the surfactant concentration from 100 to 200mM DODMAC, at a constant cosurfactant to surfactant ratio in the organic phase, resulted in a moderate increase in the extraction efficiency. The effect of the volume ratio and of the initial lysozyme concentration in the aqueous phase indicates that there are upper operating limits for the reverse micellar extraction method. These limits are determined by the saturation limit and the volume ratio between the aqueous and the reverse micellar phases. (C) 2003 Elsevier Science B.V. All rights reserved.