The active component of aurothioglucose (ATG) in effecting changes in plasma sulfhydryl (SH) levels and plasma SH reactivity with 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) was determined. These two measurements are applied clinically to rheumatoid arthritis patients undergoing chrysotherapy. Normal rats were injected intramuscularly daily for seven days with 30 mumol of either ATG or sodium thioglucose (STG)/kg body wt or with an equivalent volume of the carrier, 0.05% benzyl alcohol. ATG but not STG significantly increased total SH levels in plasma, liver, and kidney. The seven-day treatment with ATG significantly increased glutathione levels in kidney but not in liver or plasma. Thus, gold(I) rather than thioglucose was the active moiety that affected SH levels in ATG-injected rats. In vivo, gold(I) was also the active moiety that stimulated plasma SH reactions with DTNB at pH 7.4, since injection of ATG but not STG stimulated the SH reactivity in fresh plasma. In vitro, ATG increased the rate of plasma reaction with DTNB at pH 7.4, thus, gold(I) ions acted as a catalyst in the SH-disulfide exchange reaction. This study demonstrates that gold(I) but not its thiol ligand strongly interacts with protein SH groups in the rat tissues. Such an interaction may play an important role in the biological actions of gold.