Our work attempts to define the role of protein concentration on ischemic kidneys perfused under hypothermic pulsatile perfusion. Three groups of animals with ischemic, perfused, and autotransplanted kidneys were studied. All kidneys were subjected to 30 min of warm ischemia (37 C) prior to 24 hr of hypothermic pulsatile perfusion. The perfusate was silica gel fraction (SGF) of plasma, which was modified in the different groups by altering the protein concentration. Group 1 was perfused with SGF with a total protein concentration of 3.5 g/ 100 ml, group 2 with SGF at a protein concentration of 5.0 g/ 100 ml, and group 3 with SGF at a protein concentration of 7.0 g/100 ml. Group 3 kidneys perfused with the highest protein concentration showed normal function within the 1st week after transplantation and no evidence of histological injury. All animals survived for 20 days after transplantation. Group 2 kidneys perfused with SGF at a moderate protein concentration showed partial functional and histological protection, but not as good as the one observed in the high protein group. Group 1 kidneys perfused with the lowest protein concentration showed significant functional and histological damage. Only 2 of 10 animals had evidence of normal function after transplantation. Histologically, there was consistent evidence of tubular and glomerular damage. Thus, this study demonstrates that the perfusate protein concentration plays a definite role in the protection of acute post-transplant renal failure after hypothermic pulsatile perfusion.