Rat liver microsomal glutathione transferase (MGST1) is activated by limited proteolysis. Recently we purified a protease, hepsin, from rat liver microsomes that activates MGST1. In the present study the mechanism of MGST1 activation by hepsin was investigated. When MGST1 and hepsin were incubated at room temperature, MGST1 activity was markedly increased and the increase was decreased to the control level by further incubation with disulfide bond reducing agent dithiothreitol. MGST1 dimer was detected by electrophoresis after treatment of MGST1 with hepsin, instead of proteolytic product. MGST1 dimer formation accompanied by an increase in MGST1 activity was observed even in the presence of the protease inhibitor benzamidine. Furthermore, prolonged incubation of both enzymes caused the formation of MGST1 dimer and its proteolytic product. These results clearly show that the protease hepsin stimulates disulfide-linked MGST1 dimer formation resulting in activation of MGST1 and preferential degradation of MGST1 dimer. Since hepsin contains disulfide bonds in the scavenger receptor cysteine-rich (SRCR) domain, it was suggested that the SRCR domain interacts with MGST1 leading to thiol/disulfide exchange between the two enzymes followed by disulfide-linked MGST1 dimer formation.