Effect of added calcium ions on the relative milk clotting activities of porcine pepsin, Mucor miehei protease, Endothia parasitica protease, and Mucor pusillus protease was compared with that of rennin. Skimmilk, maintained at pH 6. 3 with 0.000% to 0.030% added calcium chloride, was used as a substrate. The coagulation activity of Mucor pusillus protease appeared to be most sensitive to calcium ions, followed by Mucor rniehei protease, porcine pepsin, rennin, and Endothia parasitica protease. The clotting activities of Mucor pusillus protease were also more sensitive than rennin to added calcium ions in milk samples maintained at pH values of 6.4, 6.5, and 6.6. Mucor pusillus protease and rennin were standardized to equal activities in skimmilk maintained at pH 6.3 and 6.6 containing 0.020% added CaCl2. For skimmilk maintained at pH 6.3, Mucor pusillus protease concentrations ix that gave the same clotting times as rennin in skimmilk containing 0.000% and 0.010% added CaCl2, were 9.6% and 4.5% higher than its standardized concentration. While at pH 6.6 Mucor pusillus protease concentrations were 13.2% and 5.8% higher.