This work describes the extraction and back-extraction of a lipase from crude extract of Penicillium citrinum using AOT reversed micelles in isooctane. The effect of pH, ionic strength, AOT concentration on the protein forward and backward transfer at 20 degrees C was studied. The maximum protein forward extraction (32.0%) was achieved at pH 4.0 with a 50 mmol dm(-3) acetate buffer containing 100 mmol dm(-3) KCl and 100 mmol dm(-3) AOT in isooctane. Proteins were back-extracted (82.7%) to a new aqueous phase containing 100 mmol dm(-3) pH 8.0 phosphate buffer and 1000 mmol dm(-3) KCl. No enzyme activity could be detected either in the micellar phase or in the aqueous phase after protein back-extraction. However, the lipolytic activity was recovered after hydrophobic interaction chromatography on a Phenyl Superose column. The yield obtained for the overall process was 68% for activity, 26.4% for protein recovery and the purification factor was 810-fold. A single protein band at 33 000 Da was obtained for SDS-PAGE analysis for the recovered and purified enzyme.