The fluorescence spectra of alpha-chymotrypsin were measured in aqueous-organic mixed solvents over a wide range of solvent composition. The tryptophan fluorophores were used as probes of the environmental polarity, and the structural changes of the enzyme were detected as changes of the difference in the emission wavelength between the enzyme and N-acetyl-L-tryptophan ethyl ester (Delta lambda(em)). It was found that Delta lambda(em) correlated well with the catalytic activity of the enzyme, as expressed by the hydrolysis rate of N-acetyl-L-tyrosine ethyl ester. A linear relationship was established between the hydrolysis rate and Delta lambda(em) in aqueous acetonitrile, ethanol, THF, 1,4-dioxane, and DMF with high correlation coefficients.