CARBOXYPEPTIDASE is a metallo-enzyme which contains zinc. It has been reported by Coleman and Vallee1 that the enzyme can be de-activated by removal of zinc and that activity can be restored by the addition of zinc, cobaltous, ferrous, nickel and manganous ions but not by addition of cadmium, magnesium or calcium ions. This metallo-enzyme therefore falls in the group of enzymes which have no specific metal requirement for activity2. In biological systems, however, a particular metal, zinc, is associated with the enzyme presumably because complex ion equilibria favour this particular association over all others. The situation is not very different from that found in the case of the enzyme enolase, where magnesium and manganous are the particular ions associated with activity in the biological system, but where other cations such as zinc can activate the enzyme in vitro 3. The apparent stability constants for the association of zinc and cobaltous ions with carboxypeptidase are 2 × 108 and 1 × 106 respectively1. The pH range of activation by the different cations indicates that association with the active centre is in the order zinc (II) > nickel(II) and cobalt (II). The study of complexes of a wide variety of simple organic ligands has shown2 that this stability sequence is observed only when one of the co-ordinating centres is sulphide, R—S −. I conclude that zinc is bound by a sulphide group in carboxypeptidase and give the following additional evidence in support of this binding.