1. Alkali (pH 7.6)-denatured pepsins from swine, cattle, and guinea pigs precipitate in swine pepsin antiserum. Similarly treated pepsins from the rabbit, chicken, and shark do not.
2. Pepsin antisera react with both pepsin and pepsinogen, but do not react with the serum proteins from the homologous species.
3. Pepsinogen antisera react with pepsinogen, but not with twice crystallized pepsin, nor with the serum proteins from the homologous species. Positive reactions between activated pepsinogen and pepsinogen antiserum have been observed. It was possible to remove the reacting material from either the pepsinogen or the activated pepsinogen mixture.
4. Antisera made with serum proteins do not react with the homologous pepsin or pepsinogen.