To seek novel applications of molten slag from industrial wastes, we examined influences of the slag for microbial growth.For Escherichia coli, addition of the slag inhibited the growth by 80% under static culture condition though it had no significant effect under shake culture condition.Similarly, growth of aquatic microbes from fish aquariums was also inhibited by 80% by addition of the slag under static culture condition.Next, we tested whether the slag is also available as preservative for aquariums with fishes and vases with cut flowers.When the slags were added in the aquarium with neon tetras, the occurrence of algae was not observed during three months.And also, addition of the slags showed antiseptic effect for vase water with cut chrysanthemum flowers, and the vase lives were extended to 1.7 times longer than control vases.