UW solution and HTK solution are both used for cold preservation of liver allografts. Although they are about equally effective, their compositions are very different, and they were formulated using different rationales. The authors recently showed an important role for MMPs in liver preservation injury and consequently postulated that these preservation solutions contain cryptic inhibitors of MMP activity. To determine this possibility, the ability of these solutions to inhibit MMP activity was studied. The source of MMP2 and MMP9 was human liver effluents obtained at the time of liver transplantation or commercially available human recombinant MMP2 and MMP9. MMP2 and MMP9 showed gelatinolytic activity at 37°C and also at 4°C, although activity at 4°C was reduced. Activity was inhibited by University of Wisconsin (UW) and Histidine/Tryptophan/Ketoglutarate (HTK) solutions. Examination of individual ingredients disclosed that reduced glutathione (GSH) and lactobionate in UW solution and histidine in HTK solution were the cryptic inhibitors. HTK solution was a more effective inhibitor than UW solution. GSH inhibited the activity of both enzymes, but was a much more effective inhibitor of MMP9 than MMP2. Oxidized glutathione(GSSG) was a much less effective inhibitor of the enzymes. The inhibitor constants (K(i)) of GSH for MMP2 and MMP9 were 34 μmol/L and 3 μmol/L, respectively. The authors conclude that MMP inhibition is a cryptic property of both commonly used liver preservation solutions and contributes importantly to their action. Furthermore, GSH appears to be an effective inhibitor of gelatinases at concentrations at which it is normally present in extracellular fluid.