In the period 1964-74 four different dialysate delivery systems have been used in our department. 1) Central mixing of dialysate using tap water and a dialysate delivery line with "dead ends" resulting in stagnant dialysate. 2) Central mixing of dialysate with cold distilled water, otherwise equal to system 1. 3) Local mixing of dialysate with cold distilled water, delivered through a line with "dead ends" resulting in stagnant water. 4) Local mixing of dialysate with distilled water, cooled to 25 degrees C just prior to use, reduced "dead ends" and monitored constant overflow to drain through the water supply line. The bacterial contamination of the four systems was examined and related to the clinical occurrence of pyrogenic and other reactions. An improvement was noted with the change from central to local mixing of dialysate (system 3) but complete sterility and virtual freedom from clinical reactions were first obtained in system 4. It is concluded that the use of sterile or near sterile dialysate is recommendable.