The cytotoxicity of musty odor-emitting substances, geosmin (GM) and 2-methylisoborneol, at a concentration of 10 ng/L - 300mg/L was investigated using cultured mammalian cells. These two compounds exhibited no cytotoxicity in either the colony-formation of human KB cells or WST-1 assays of human -, monkey -, and dog -derived cells. These results suggest that the maximum concentration (700 ng/L) of GM found in the water of Lake Shinji is not toxic.