Isoleucine is known to be a very effective inhibitor and stabilizer of threonine deaminase. In contrast, valine has been reported to be a positive effector for the enzyme by action on a separate site. However, the apparent increase in activity caused by valine is due to stabilization of the enzyme. When stabilization is accomplished by other means, valine exerts only an inhibitory effect. Thus, valine, like isoleucine, both stabilizes and inhibits threonine deaminase and may well do so via the isoleucine site on the enzyme.