The mechanism of the protective action of acetate against heat-inactivation of the L-amino acid oxidase (L-amino acid:oxygen oxidoreductase (deaminating, EC 1.4.3.2) of Crotalus adamanteus venom has been investigated. The entropy of activation for the heat-inactivation of the enzyme in the presence of acetate was 224 e.u. compared to 114 e.u. in its absence. The Km value of the enzyme for L-arginine was decreased from 3. 86 mM to 0. 77 mM in the presence of 330mM acetate. The Vmax was reduced from 227 to 161 ㎕O₂ consumed per hour per ㎎ of enzyme protein. These results are taken to mean that the protective action of acetate is by preventing the enzyme protein from assuming a less ordered state, at high pH, by combining at a site different from the substrate-binding site.