A new method in which phenolphthalein monophosphate is used as substrate has been developed for determining residual alkaline phosphatase: Upon hydrolysis, phenolphthalein is released and yields a red solution that is compared visually with a standard prepared from the same milk. Seven collaborators made 168 observations that were used in a statistical study of the results. Chi-square calculations and tests of independence indicated that the described method yields results on skim milk, whole milk, and light cream that are as precise and either as accurate or more accurate than those obtained with the Scharer I method. Based on these results, it is recommended that the method be adopted as official, first action.