1. 1. The reversible inactivation of l-amino acid oxidase, previously described for the enzyme of moccasin venom, occurs in all species of Crotalidae venoms and in many Viperidae venoms, while Elapidae do not at all undergo this type of inactivation. This indicates a basic difference in the protein component of the enzyme from Crotalidae and from Elapidae. 2. 2. The l-amino acid oxidase activity of 23 species of venoms has been determined under conditions where the enzymes are stable. 3. 3. The following univalent anions protect the enzyme from inactivation to about the same extent: Cl −, Br −, F −, NO 3 +, CNS −, N 3 +, and ClO 4 +; I −, which is somewhat inhibitory to the enzyme, protects less effectively. The singly charged phosphate, metaphosphate, and arsenate anions do not prevent the inactivation. None of the doubly or triply charged anions or undissociated acids tested were protective. The cationic constituents of buffers are without effect on the enzyme at the concentrations tested. 4. 4. In dilute solutions of phosphate and related anions the inactivation and its reversal proceed in the same manner as in distilled water. Phosphate enhances the extent of inactivation only at high concentrations. 5. 5. The effect of the protective univalent anions is on the stability of the enzyme; they are not required for activity. The differences in the behavior of various anions are ascribed to selective binding of certain anions by the protein at those sites where bonds are ruptured during the reversible inactivation process.