PURPOSE: To readily select cells of highly producing physiologically active substance by using a specific method of raising ability of the cells for reducing a tetrazolium compound by adding a physiologically active substance to mouse leukemia cells.

CONSTITUTION: Firstly, mouse leukemia cells M1 are treated with a supernatant liquid of cells of producing a physiologically active substance so that ability of the cells M1 for reducing a tetrazolium compound [preferably 3-(4,5- dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] added is raised. Then, absorbance of a formazan compound formed from the tetrazolium compound by the raised reducing ability of the cells M1 is measured to determine the physiologically active substance in the supernatant liquid of culture.