We report an improved kinetic colorimetric system for measuring lactate dehydrogenase activity in serum. In the system a tetrazolium salt, 2-p-iodophenyl-3-p-nitrophenyl-5-phenyl tetrazolium chloride, is used as the chromogenic indicator of dehydrogenase activity, with diaphorase serving as the electron transfer agent. All ingredients required for an assay are combined in a single dry reagent that is stable at room temperature. The method is 2.5 times as sensitive as the ultraviolet method of Wacker and has a dynamic range three times that of the ultraviolet method. Reducing substances in serum do not affect the results. Precision, range of linearity, and stability of reagent after reconstitution are excellent. Results for fresh sera correlated well with those obtained by the "A-Gent" ultraviolet method (Wacker method at 37°C) and with the SMA 12/60.