IN a previous communication it has been shown by Basu and Nandi1 that the extent of inactivation consequent on the unfolding of the protein chain of an enzyme molecule could be detected by iodimetric titration of the SH-groups uncovered by the unfolding of the coiled-up molecules. In this connexion it has been noted that increase in iodine absorption occurs in cases where the enzyme has been partially deactivated, although no denatured, insoluble protein was formed. It may be concluded, therefore, that denaturation leading to insolubilization takes place only when the unfolding of the protein coil has proceeded to an appreciable extent, whereas deactivation makes its appearance much earlier and with a lesser degree of uncoiling. These observations suggest that the variation in the extent of enzyme activity at different pH's may, to some extent, depend on the extent of coiling or uncoiling of the enzyme molecule. In order to test this point, pepsin solution was adjusted to different pH's by the addition of the necessary amount of hydrochloric acid or sodium hydroxide as the case may be, and titrated with N/2,000 iodine solution below 15° C., using starch as indicator. The results are shown in Table 1.