 The purposes of this study are to determine if the performance of a dialyzer depends on how tightly the hollow fiber membranes are packed in the housing of the dialyzer the membrane packing density and if that dependence is a function of the dialysate flow rate. The study will examine how efficiently three different sized molecules pass through a dialyzer membrane at different dialysate flow rates in dialyzers with different membrane packing densities. Transfer of urea, phosphorus and beta-2-microglobulin from blood to dialysate will be measured during clinical hemodialysis using four different dialyzers, each used at three different dialysate flow rates. The data derived from these measurements may provide insight into the importance of membrane packing density as a design parameter for hemodialyzers and if changing the membrane packing density might provide equivalent performance at a lower dialysate flow rate. 