Silver-containing zeolite, which is natural or synthesized zeolite processed by ion exchange with silver, has an antimicrobial nature. Because of its long-lasting antibacterial activity, this substance is widely used as a plastic additive for structural parts of kitchen utensils, household electrical appliances, toilet seats and telephone handsets. With its antibacterial activity and character of filter medium, this substance is also suitable for use in simple water purification systems, and its practical use in this field is now being expected. Although silver-containing zeolite is non-carcinogenic, it has chronic toxicity, with the acceptable daily intake being calculated as 11 mg/kg・day for type A zeolite processed by silver-zinc-ammonium complex substitution. When used in water-purifying devices, silver-containing zeolite comes in direct contact with water. Therfore, serious attention from the viewpoint of public health should be paid to the possibility of elution of silver or other substituent factors and zeolite components. In this connection, we carried out elution tests of silver-containing zeolite itself, plastic products containing this type of zeolite as an additive and silver utensils. The amounts of cations of silver and other factors eluted from silver mordenite (Table 3), silver-ammonium mordenite (Table 4) and silver-zinc-ammonium type A zeolite (Table 5) immersed in 1L of water for 24 hours were determined. The types of water tested were purified water and tap water supplied in Tokyo, Osaka and Nagoya. The tap water varied in cation content according to the location (Table 2), with the total cation level being highest, 2.95 meq/L in Tokyo (Table 9). Elution of silver was always under the detection limit of 0.01 ppm, except that a trace amount of silver was detected with silver-containing mordenites immersed in the tap water of Tokyo. Zinc and ammonium were eluted increasingly, to a maximum concentration of 0.59 ppm and 5.2 ppm, respectively, as the total cation level in water increased. The mechanism of their elution was considered to be substitution by other cations present in tap water. There was not even any trace amout of eluted silver when zinc coexisted, suggesting that substiturion by inhibited the elution of silver. Concentrations of silica, aluminum and sodium in eluate were the same as those in original tap water, indicating that zeolite itself was not eluted. The allowable concentration of silver in tap water was set at 0.05 mg/L in the former Soviet Union. The corresponding standard for zinc is 1 mg/L in Japan. Elution of silver from plastic products with silver-containing zeolite and silver utensils was under the detection limit of 0.01 mg/L (Tables 6 and 7). There was also no elution of silver in tap water on flow tests (2L/min) (Table 8). Thus, silver was found to be firmly bound to zeolite in the silver-containing zeolite examined. Elution of silver was very slight if any, and was suggested to be related to the order of selectivity in ion exchange. The coexistence of harmless substituent cations which are at lower ranks in selectivity than silver seems to be effective for securely inhibiting the elution of silver.