PURPOSE: To accomplish quick detection of the mutagenicity of environmental carcinogens found in waters, the air, etc., based on β-galactosidase activity determined on the basis of the optical density of a specific enzymatic reaction solution.

CONSTITUTION: Firstly, (A) Salmonella bacteria transformed by plasmid having umuC gene is put to culture in the presence of (B) (1) a drug metabolically active substance for mammals and (2) a substance to be detected to produce a culture solution 0.1-0.4 in OD600. This culture solution is subjected to cell wall destruction treatment and mixed with an aqueous solution of (C) 2- nitrophenyl-β-D-galactopyranoside to make and enzymatic reaction, and based on the optical density of the resulting enzymatic reaction solution, β- galactosidase activity is determined, and on the basis of this activity, the mutagenicity of the above-mentioned substance to be detected is detected. The amount of the culture solution should be 0.5-5.0ml per mg of the component C.