In a method for testing the Ag-sensitivity of bacteria in water disinfection, AgNO3 at various concns. is added to the water and bacterial growth is monitored for several days. The detn. of a toxic limiting Ag concn. to est. the oligodynamic effects on bacteria in tap water showed that to preserve water samples for 1 day, 20 mg/L suffice, but 7-day preservation requires a dose of 50 mg/L. Addn. of Ag caused changes in the species distribution in favor of Ag-tolerant species, suggesting that ion exchangers using Ag as biocide must be regularly checked.