1. A method has been described for isolating a crystalline protein with high proteolytic activity from bovine gastric juice by means of precipitation with magnesium sulfate and fractionation of the precipitate with acetone and magnesium sulfate.
2. The crystalline protein obtained in this way has the same crystalline form, optical activity, and specific activity, as determined by a number of methods, as does the crystalline protein previously isolated from swine gastric mucosa.
3. The solubility of the two preparations, however, is additive so that they are different although very closely related proteins.