A model was proposed to describe the effects of the main factors in aqueous-organic two-liquid-phase media on the stability of papain. The relationships between the half-life of papain activity and these factors including interfacial tension, stinting rate, phase volume ratio and temperature were investigated. The results showed that these factors had notable effects on papain stability except temperature. The correlation coefficient between the model and the experimental data were 0.829, which indicated the model is practicable. (C) 2001 Elsevier Science Inc. All rights reserved.