As the quality of water in the dialysis fluid varies considerably, and in view of the fact that endotoxin or active derivates can cause acute and chronic side effects in patients under chronic hemodialysis treatment, the dialysis fluid must be sterile and endotoxin free. The predialyzer fluid in 20 hemodialysis patients was investigated. The bacterial load was between 5/ml and 12,000/ml, the endotoxin concentration was high and extremely variable. Therefore, we introduced the ultrafiltration of the dialysis fluid by a polyamide hollow fiber membrane before entering the dialyzer. All samples were free of bacteria, and the concentration of endotoxin was lower than the detectable limit. With this procedure, we can obtain sterile dialysis fluid, which is endotoxin free. Our preliminary results showed that interleukin-1 in the patients was significantly (p < 0.005) lower under ultrafiltration of the dialysis fluid than without ultrafiltration.