High levels of magnesium can reverse the anticoagulation of cryoprecipitated plasma (CPP), resulting in fibrin formation. When pig cadaver kidneys were flushed with Collins' C2 solution containing a high concentration of magnesium, clotting of CPP perfusate occurred when the CPP initial magnesium level was high, but not with low magnesium levels. Flushing with C2 and preservation with an albumin perfusate, or flushing with Ringer's solution before CPP perfusion, did not result in fibrin formation. Small amounts of heparin added to CPP did not prevent fibrin formation. C2-preserved kidneys should not be additionally preserved with CPP perfusion preservation without special consideration of magnesium levels.