An enzyme catalyzing thiol-disulfide interchange of glutathione and disulfides and the reaction between glutathione and thiosulfate esters has been purified 40 000-fold from rat liver cytosol. The enzyme, named thioltransferase (Askelöf, P., Axelsson, K., Eriksson, S., & Mannervik, B. (1974) FEBS Lett. 38, 263--267), was homogeneous in several electrophoretic systems, had an isoelectric point at pH 9.6, and contained 8.6% (w/w) carbohydrate. The catalytic activity had a distinct optimum at pH 7.5. A series of substrates was tested at a constant glutathione level; the kcat values (at 4mM glutathione) were all in the range of about 10(4) min-1. The substrates included mixed disulfides of glutathione, other low-molecular-weight disulfides, S-sulfocysteine and S-sulfoglutathione, and peptide disulfides such as insulin, oxytocin, ribonuclease, and the mixed disulfide of glutathione and egg-white lysozyme. The enzymatic reaction was inhibited by an excess of glutathione (greater than 4mM).