The formation of non-covalent complexes with polyelectrolytes has been suggested to enhance the resistance of enzymes towards inactivation by organic solvents in their homogeneous mixtures with water. Existence of such complexes in water-cosolvent media was proved by experiments with a fluorescence dye, eosin. In the case of catalysis by alpha-chymotrypsin, formation of the complex with polyelectrolytes produced two major effects : i) considerable increase in enzyme activity at concentrations of ethanol and N,N-dimethylformamide of 10-30 % v/v; ii) conservation of the enzymatic activity at cosolvent concentrations ofmore than 40% v/v, where the native enzyme is completely inactive. General character of the observed activation and stabilization phenomena was shown by example of several experimental systems.