The objective was to standardize an enzymatic method for measurement of milk urea nitrogen (MUN) and determine the within and between laboratory performance of the method by conducting an interlaboratory study. In the first step of the method, urea + water are converted to ammonia and CO₂. In the presence of glutamate dehydrogenase (GIDH) and reduced NADPH, ammonia (as ammonium ions: NH₄⁺) reacts with 2-oxoglutarate to form l-glutamic acid and NADP⁺. The amount of NADP⁺ formed is stoichiometric with the amount of ammonia. For each mole of urea, 2 moles of NADPH are consumed. This is measured by the decrease in light absorbance at 340 nm. In November and December 2018 and January 2019, 9 labs tested 14 different milks in duplicate each month. Data were analyzed to remove statistical outliers using the Cochran and single and double Grubbs outlier tests. For November, December, and January 38, 8, and 6 statistical outliers, respectively, were removed before calculation of method performance statistics. With time and experience of using the method the number of statistical outliers has decreased. Within (S_r) and between (S_R) laboratory method performance statistics for the MUN reference method in November and December 2018 and January 2019 were S_r = 0.076, 0.083, and 0.069, S_R = 0.138, 0.121, and 0.104, RSD_r = 0.465, 0.502, and 0.395, RSD_R = 0.843, 0.736, and 0.595, r-value = 0.213, 0.231, and 0.193, R-value 0.385, 0.339, and 0.291, respectively. With time and experience the between laboratory performance of the method is improving. An official interlaboratory study of the method will be conducted to support Association of Official Analytical Chemists approval of the method for use as a reference method for calibration of infrared milk analyzers.