Abstract: A serum sample is added to a reaction liquid obtained by adding an aqueous solution of a mixture of a tetrazolium compound and phenazine methosulfate to an enzyme-coenzyme reagent comprising LPL, GDH and NAD thereby effecting hydrolysis reaction of serum triglycerides and subsequent dehydrogenation reaction of resulting glycerol and the liquid after the reaction is subjected to colorimetry in a visible ray region to determine the triglyceride content of the serum.