1. Solubility curves of crude pepsin preparations indicate the presence of more than one protein.
2. One of these proteins has been isolated and crystallized and found to have constant activity and constant solubility in several solvents.
3. The solubility measurements are complicated by the unstable nature of the protein and the fact that in certain solvents the solubility of the protein is markedly affected by the presence of non-protein nitrogen decomposition products while in others this is not the case.
4. A more insoluble protein has been prepared of lower solubility and lower activity, as measured by hemoglobin digestion. The activity, as measured by the digestion of other proteins, is about the same as the more soluble fraction. This insoluble fraction does not have constant solubility.
5. Mixtures of the insoluble and the soluble fractions give preparations having rounded solubility curves typical of solid solutions and resembling very closely those of the original preparation.
6. A small amount of pepsinogen and pepsin from pepsinogen has been separated which has nearly twice the enzymatic activity on hemoglobin as does the most active pepsin previously isolated.