Summary o 1. A new series of thiols, N-mercaptoacetyl derivatives of substituted anilines, have been found to be activators of papain. 2. Mass-action expressions have been derived on the basis of a modification of a previously proposed mechanism for the activation of inactive papain by thiols. 3. The derived mass-action expressions are consistent with the data obtained by activating inactive papain in the presence of disulfide.