This Communication reports the roles of cysteine, lysine and arginine residues inchicken liver fructose-6-phosphate-2--kinase. Chemical modification of the enzyme withDTNB demonstrates that there are nine SH groups in the enzyme. Among these SH groups, atneutral pH six SHs can be titrated; in the presence of Fru6P four can be titrated; and inthe presence of ATP seven can be titrated. During the process of titration, the activity ofthe enzyme increases first and subsequently decreases gradually to the original level. NENIhas an effect similar to DTNB for the activity changes of the enzyme. On the contrary,PCMB or iodoacetic acid has a little effect on the activity of the enzyme. The pH profileof the activity shows that there is an essential ionizable group with a pK_a of 9.0, mostprobably the lysyl residue, which responds to the catalytical reaction. PLP or phenylglyoxalinactivates the enzyme. For PLP inactivation, Fru6P, one of the substrates, most effectivelyprotects the enzyme. Both products Fru2,6P_2