Transketolase from baker's yeast is rapidly inactivated in the presence of N-acetylimidazole. According to kinetic data, acetylation of one amino acid residue of the protein per active site is sufficient for TK* inactivation. The holoenzyme is inhibited more slowly than is apotransketolase. The presence of a tyrosine residue in the enzyme's active site, essential for activity, is suggested.