Papain activity was studied in water-organic solvent mixtures using the fluorogenic substrate Dabcyl-Lys-Phe-Gly-Gly-Ala-Ala-Edans. The increase of organic solvent (MeOH, EtOH, iPrOH, TFE, MeCN, (MeO)(2)Et and DMF) concentration in the mixture caused a substantial decrease the initial rate of papain-catalyzed hydrolysis. Moreover, the number of papain active sites decreased with the increase of DMT and MeOH concentration.