The combined effect of solvent nature and aqueous medium composition on the efficiency of a whole-cell biocatalyst in organic solvent-containing systems was evaluated, using the side-chain cleavage of beta-sitosterol by free mycobacterial cells as model system. Trials were performed in two-phase liquid systems and in solvent-saturated aqueous systems. Aqueous phase composition influenced solvent distribution between the cells and the medium and, hence, catalytic activity. The resulting partition coefficient was different for active and autoclaved cells, suggesting active solvent incorporation. This coefficient was used to estimate the solvent concentration in the cells at system saturation and led to the identification of systems for which high solvent concentration in the cells apparently favoured catalytic activity. (C) 2003 Elsevier Inc. All rights reserved.