Isoenzymes of N-acetyl-beta-glucosaminidase can be separated by electrofocusing in thin layers of polyacrylamide gel. Patterns of isoenzymes from human liver tissue and serum show changes in liver diseases. Preliminary results indicate, that N-acetyl-beta-glucosaminidase isoenzyme A is increased in liver tissue from patients with chronic liver diseases in relation to the activity of mesenchyme reaction. The described method will be discussed in comparison with known separation techniques.