Acetolactate synthase purified from Serratia marcescens ATCC 25419 was rapidly inactivated by the thiol specific reagent p-chloromercuribenzoate (PCMB), the tryptophan specific reagent N-bromosuccinimide (NBS), and the arginine modifying reagent phenylglyoxal (PGO). Inactivation by PCMB was prevented by both ${alpha}$-ketobutyrate and pyruvate, and the second order rate constant for the inactivation was $2480;M^{-1}{cdot}min^{-1}$. The reaction order with respect to PCMB was 0.94. The inactivation of the enzyme by NBS was also substantially reduced by both ${alpha}$-ketobutyrate and pyruvate. The second order rate constant for inactivation by NBS was $15,000;M^{-1}{cdot}min^{-1}$, and the reaction order was 2.0. On the other hand, inactivation by PGO was partially prevented by ${alpha}$-ketobutyrate, but not by pyruvate. The second order rate constant for the inactivation was $1480;M^{-1}{cdot}min^{-1}$ and the order of reaction with respect to PGO was 0.75. These results suggest that essential cysteine, tryptophan and arginine are located at or near the substrate binding site.