This part of ISO 11816/IDF 155 specifies a fluorimetric method for the determination of alkaline phosphatase (ALP, EC 220.127.116.11) activity in raw and heat-treated whole milk, semi-skimmed milk, skimmed milk and flavoured milks. The method is applicable to milk and milk-based drinks from cows, sheep and goats. It is also applicable to milk powder after reconstitution. The instrument can read activities up to 7 000 milliunits per litre (mU/l). If the activity is higher than 7 000 mU/l, it is diluted with alkaline phosphatase-free milk so as to obtain a measurement not higher than 7 000 mU/l. The alkaline phosphatase activity of the sample is measured by a continuous fluorimetric direct kinetic assay. A non-fluorescent aromatic monophosphoric ester substrate, 2'-[2-benzothiazolyl]-6'-hydroxybenzothiazole phosphate, in the presence of any alkaline phosphatase derived from the sample, undergoes hydrolysis of its phosphate radical, producing a highly fluorescent product. Fluorimetric measurement of alkaline phosphatase (ALP) activity is measured at 38 °C over a 3-min period. This includes pre-incubation of substrate and sample, followed by multiple kinetic readings of the reaction rate. This document supersedes DIN EN ISO 11816-1:2006-07. Compared with this standard, the scope has been revised to consider phosphatase activities in raw milk higher than 7 000 mU/l and the subclauses concerning A/D tests and positive and negative control tests have been revised. A clause "Reagent controls to test the suitability of ready-to-use working substrate" has been added and precision data have been updated. Furthermore, the results from two collaborative trials completed in 2008 and 2010 have been incorporated. The text has been technically and editorially revised and brought in line with current rules of presentation. This document (EN ISO 11816-1:2013) has been prepared by Technical Committee ISO/TC 34 "Food products" (secretariat: AFNOR, France), Subcommittee SC 5 "Milk and milk products" (secretariat: NEN, Netherlands) in collaboration with Technical Committee CEN/TC 302 "Milk and milk products - Methods of sampling and analysis", the secretariat of which is held by NEN (Netherlands). The national committee responsible for this standard is NA 057-05-13 AA "Milch und Milchprodukte - Probenahme- und Untersuchungsverfahren" ("Milk and milk products - Methods of sampling and analysis ") at DIN.