This International Standard specifies the determination of phytase activity in feeding stuff samples, including feed raw materials from plant origin, compound feeds (complete, complementary, mineral feeds), premixtures and feed additives. The method is collaboratively validated for the determination of phytase activity in complete feed, complementary feed including mineral feed, premixtures and feed additives. The method does not distinguish between phytase added as a feed additive and endogenous phytase already present in the feed materials. Therefore, the method is also applicable for feed materials from plant origin. The method cannot be used to evaluate or compare the in vivo efficacy of the phytase product. It is not a predictive method of the in vivo efficacy of phytases present on the market as they can develop different in vivo efficacy per unit of activity. Note 1 Products which are not included in the validation studies in the Annexes have to be checked especially on their linearity of response (release of phosphorus)