This International Standard specifies the determination of phytase activity in feed samples. The method does not distinguish between phytase added as a feed additive and endogenous phytase already present in the feed materials. 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. The method is suitable and validated exclusively for the determination of phytase activity and exclusively in complete feeds. NOTE The harmonized method was developed on the basis of the presently existing phytase products [E1600 (EC 3.1.3.8, 3-phytase), E1614 (EC 3.1.3.26, 4-phytase), and E1640 (EC 3.1.3.26, 4-phytase)]. Therefore, it might not necessarily be suitable as such for phytase products that are developed in the future. The harmonized method is thus a tool which is useful only to evaluate the total phytase activity in feed samples.