The preparation, regeneration and UV mutagenesis of protoplasts of strain S-12-86, which had high lysozyme producing activity, were studied. The results showed that the optimal conditions of the preparation and regeneration of protoplast were as follows: incubation time of the strain S 12 86 was 18 h, concentrations of lysozyme was 1.0 mg/ml, and time of enzymolysis was 30 min at 35 ℃. The rates of formation and regeneration of protoplast under the optimal conditions were 97.6% and 23.6%, respectively. The optimal condition for mutagenesis of protoplasts was achieved when the protoplasts of strain S-12-86 were irradiated by a 30 w UV lamp at an 80 cm distance for 120 s. After being selected from a large amount of the regenerative mutants, a genetically stable mutant strain R-J-101, of which enzyme activity was 40% higher than that of the original strain S-12-86, was obtained under specific mutation conditions.