A disposable fluid circuit kit for extracorporeal blood treatment systems that supply replacement fluid to a patient is described. The circuit has a blood portion that carries blood and subjects it to some treatment, such as dialysis or hemofiltration. A portion carries filtrate or dialysate. A replacement fluid portion that carries fluid used to compensate for fluid loss is connectable to a supply of replacement fluid. a sterilizing filter in the replacement fluid portion sterilizes the replacement fluid before introduction into the patient acting as a guard against touch contamination of the connected replacement fluid container. Preferably, the filter has a pore size effective to eliminate pyrogens.