Mucor-rennin, the crystalline milk-clotting enzyme, isolated from Mucor pusillus var. Lindt, has an acid protease activity. The optimum pH for the digestion of k-casein is 4.5, while that for hemoglobin digestion is 4.0. The skim milk solution was easily clotted acidic solution than alkalin solution, and the milk clotting activated by Ca ion. The enzyme was heat stable against heat from pH 4.0 to 6.0 but was more stable at pH 5.0. The activity of the enzyme at pH 5.0 did not decrease at 30 C for 15 days and the activity was not effected by sodium propionate and salicilic acid. Therefore, the enzyme of liguid type could store for a long time and could be transported from Erzyme production Co. to Manufacture of cheese Co. by adding the antiesptic and by adjusting pH to 5.0.