The bacterial concentrations of the municipal water increased by more than 39-fold when subjected to reverse osmosis; then decreased by greater than 200-fold within the reservoir and water supply system of the hemodialysis center. The bacterial concentrations of dialysate solutions in contact with proportioning single-pass artificial kidney machines were as low or lower than the water from the hemodialysis center system ( < 10 CFU/100 ml.). The complete opposite was observed in the recirculating single-pass artificial kidney machines where bacterial concentrations in the dialysate solution reached levels greater than 1.0 × 106 CFU/100 ml