Endotoxins, or fragments thereof, can reach the blood stream of dialysis patients, transported by diffusion and connection across the intact high-flux membrane. This transfer depends upon the phenomenon of back filtration. Back filtration generally occurs under conventional high-flux dialysis conditions with membranes having an ultrafiltration coefficient in blood (UF-C) above 20 ml/hr/m2/mmHg. The clinical consequences of back filtration vary from center to center depending primarily on the quality of dialysate. We therefore surveyed the bacterial and endotoxin levels of purified water and effluent dialysate in a cross section of dialysis centers in the central United States. Using a high recovery medium, we found that 53% of the centers had bacterial counts above the Association for the Advancement of Medical Instruments standard in water (20% cfu/ml) and 35% above the standard in dialysate (2,100 cfu/ml). Endotoxin concentrations higher than 5.0 EU/ml in both water and dialysate were found in 4% and 11.8% of the centers, respectively. Since high-flux membranes are believed to be of benefit for long-term dialysis patients, manufacturers will have to offer dialysate preparation systems with additional safety features. The proper membrane design will be a key to the success of such systems.