The catalytic activity, thermostability (resistance to monomolecular thermo-inactivation) and molecular mobility of chymotrypsin and trypsin mechanically entrapped into polymethacrylate and polyacrylamide gels have been studied. It has been established that the thermostability of the enzymes does not depend on the concentration of electroneutral polyacrylamide gel over the range of 0–50 w/w%. However, in polymethacrylate gel of concentration higher than 30 w/w%, when a high catalytic activity is retained, the thermostability of chymotrypsin dramatically increases: in 50 w/w% gel the first-order rate constant for thermoinactivation of the enzyme at 60°C is 10 −5 that in water. Based on these data and also on experimentally obtained results on transitional and rotational diffusion of both native and modified enzymes, the following mechanism of enzyme stabilization is formulated and proved. In principle, the protein molecule of an enzyme may form with the three-dimensional lattice of polyelectrolyte gel multiple noncovalent linkages (via electrostatic or hydrogen bonds); as a result, the structure of the enzyme becomes more rigid and its thermostability should increase. However, since these bonds are relatively weak, in diluted gels they can hardly be realized, as the “quenching” of the transitional movement of the enzyme molecules, accompanying complex formation would have required a heavy entropy loss. At the same time, in concentrated gels, this unfavourable entropy contribution is absent as the polymer's lattice provides significant steric hindrances for the transitional diffusion, so that the molecules almost stop moving. That is why weak linkages between the protein globule and the support can be realized here. That the complex formation does take place is indicated by the fact the rotational diffusion of the protein molecules is almost completely frozen. When there is no specific protein-support interaction (in polyacrylamide gel), no deceleration of the rotational movement of the protein molecules occurs and no noticeable increase in the thermostability of the enzymes is observed. It is possible that the mechanism discovered by us functions in vivo and is responsible for the stability (and, which is important, for stability regulation) of the proteins incorporated in biomembranes. On the other hand, the results obtained by us may enrich enzyme engineering, as they allow the general strategy of production of stabilized enzymes to be outlined.