Kinetic behavior of alpha-chymotrypsinin the reaction of hydrolysis of the N-acetyl-L-tyrosine derivatives was investigated in non-denaturing water-dimethylsulfoxide and water-ethanol mixtures. Similar specific interactions between the two solvents and the active site of alpha-chymotrypsin were shown to result in similar kinetic effects. It is proposed that the changes in the active site structure of the enzyme caused by the interaction with the organic solvents ("conformational isomer" formation) resulted in two parallel processes--acceleration of the acyl-enzyme formation step and a decrease in the deacylation rate. The possible molecular mechanism of this phenomenon and an adequate kinetic model describing the data are discussed.