Abstract Threonine deaminase (l-threonine hydro-lyase (deaminating), EC 4.2.1.16) from Bacillus subtilis catalyzes the deamination of threonine at a constant rate upon addition of substrate. In the presence of isoleucine, the reaction begins slowly and increases to a steady state, inhibited rate. Essentially the same steady state rate is observed when the same amount of isoleucine is added to enzyme already catalyzing the reaction but the inhibited rate is achieved slowly. In the absence of inhibitor, the substrate saturation curve is described by a hyperbola. In the presence of isoleucine, the curves are sigmoid and can be fitted by the method of Cleland to an equation containing S2 terms. Isoleucine was shown to be a competitive-type inhibitor. The steady state kinetic data can also be fitted to kinetic versions of equations which describe the allosteric model of Monod, Wyman, and Changeux for regulatory proteins. Valine antagonized the inhibition by isoleucine in a competitive fashion. It did not activate the enzyme in the absence of isoleucine, rather, it exerted a weak (15%) inhibition which was noncompetitive.