The bacteriostatic effect of very low Ag concn. (oligodynamic effect) shows a decrease in intensity after a time (within a period of several weeks), which is accompanied by an increase in the tolerance of the bacterial flora towards Ag ions. The sensitivity of the bacteria population in the drinking water supply fluctuates strongly (5 -50 mg Ag+/L as toxic concn. limit), consequently the buildup of a flora of Ag-tolerant microorganisms in Ag-treated ion exchange resin beds is further enhanced. The routine bacteriol. monitoring of ion exchange units is therefore necessary.