Abstract: An all-in-one reagent is provided for the quantitative colorimetric determination of creatine phosphokinase in blood serum or plasma wherein the tetrazolium dye, 2-(p-iodophenyl)-3-(p-nitrophenyl)-5-phenyl tetrazolium chloride (INT), preferably together with an electron transport such as diaphorase, and compatible substances which are enzymatically converted by creatine phosphokinase to an end product that will reduce said dye from its normally colorless state to a colored reduced state are combined into a homogeneous composition which can be dried to a stable reagent.