The effects of different concentrations (20-95%) of organic solvents (ethanol, 1,4-dioxane and acetonitrile) were studied on alpha -chymotrypsin and trypsin from bovine pancreas. The changes in secondary structure were followed by CD measurements, and the apparent Michaelis constants (K-M (app)) and the stabilities of the enzymes were determined. Significant alterations in the CD spectra were found for both enzymes at the different organic solvent concentrations. The apparent K-M values of trypsin and alpha -chymotrypsin decreased as the low solvent concentrations were elevated, but then increased in the presence of higher organic solvent concentrations. The stabilities of the enzymes changed on increase of the organic solvent concentration; trypsin exhibited a higher stability than that of alpha -chymotrypsin in all organic solvents. These results show that at an organic solvent content of 95% the manifestation of an enzyme activity similar to that measured in water can be attributed to the similar compositions of the secondary structural elements. 2001 Academic Press.