Roussin salt (K[Fe 4S 3(NO) 7]) reacts with chelates without destroying them, at pH values corresponding to the active range of most enzymes. Inhibition of this activity, which this complex provokes in all the enzymes studied till now, cannot be attributed to the displacement of the metal by the Roussin salt. The basic nitrogen groups react with Roussin salt in the free state, as well as in the chelate form. In both cases, the reaction products are poorly soluble. This poor solubility indicates that if these groups are found in the active centre of an enzyme, the Roussin salt would block this centre, irrespective of the state in which these groups were found.