Amount and reactivity of SH-groups in thiamine pyrophosphokinase from rat liver tissue were studied using the mercaptide-forming reagents HgCl2 and p-chloromercury-benzoate (pCMB). Two types of SH-groups were found: the first--readily accessible and the second--partially concealed groups. Amount of these groups altered in presence of 8 M urea and 0.5% sodium dodecylsulfate. The enzymatic activity was inhibited by 30% after pCMB modification of the first type SH-groups and by 100% if second type SH-groups were blocked. All the substrates used, except of ATP, protected partially thiamine pyrophosphokinase against the inactivating effect of the reagent. The mercaptide-forming reagents exhibited dissimilar efficiency apparently due to different nature of the chemical bonds formed: pCMB developed the bond protein-S-HgR and HgCl2 formed--(protein-S)2Hg bond. The data obtained suggest that the SH-groups are localized close to the active site of thiamine pyrophosphokinase and that they participate in stabilization of the molecular structure of the enzyme.