Dialysis fluids containing at least 10(7) bacteria per milliliter and as much as 12,500 ng of endotoxin equivalents per milliliter were dialyzed and ultrafiltered with three types of disposable hemodialyzers. Neither bacteria nor endotoxin, as measured by the Limulus lysate assay, was detected in the sterile compartment despite ultrafiltration. Under these favorable conditions for endotoxin transfer, the maximum transfer rate was calculated to be less than 3.5 ng of endotoxin equivalents per hour. At this rate, it is unlikely that pyrexia during hemodialysis is due to the transfer of endotoxin across an intact dialyzing membrane. Provided that the integrity of the dialyzing membrane is maintained, this investigation indicates that the risk of endotoxemia or bacteremia associated with the use of contaminated dialysis fluids is negligible.