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The effect of the ingestion of myo-inositol hexaphosphate (IP6) and phytase (EC 3.1.3.26) on the digestibility of casein was investigated using growing broiler chickens. A total of 64 female Ross broilers were used in a precision feeding study. One group of 8 birds was fed a solution of glucose to estimate endogenous losses. Seven groups, each of 8 birds, were fed either casein, casein + 1,000 units of phytase activity (FTU), casein + 2,000 FTU, casein + 0.5 g of IP6, casein + 0.5 g of IP6 + 1,000 FTU, casein + 1 g of IP6, or casein + 1 g of IP6 + 1,000 FTU. The excretion of DM, amino acids, nitrogen, minerals, and phytate-phosphorus was determined over a 48-h period and nutrient digestibility coefficients were calculated. Casein was found to be highly digestible, with true coefficients of DM, N, and amino acid digestibility of between 0.85 and 1.0. However, the ingestion of IP6 reduced (P < 0.05) the digestibility coefficients of amino acids, N, and DM of casein compared with birds fed casein alone. Supplementation of the mixture of casein and IP6 with phytase improved (P < 0.05) the digestibility coefficients of amino acids compared with birds fed on casein and IP6 with no supplemental phytase. The excretion of endogenous minerals was increased (P < 0.05) by the ingestion of IP6 and reduced (P < 0.05) by the supplementation of IP6 with phytase. In the absence of exogenous phytase, the recovery of phytate-P in excreta was approximately 80%. However, the recovery of phytate-P was significantly reduced by the addition of exogenous phytase to the IP6/casein mixture. It can be concluded that the ingestion of IP6 reduces the digestibility coefficients of amino acids and the metabolizability of nitrogen of casein. This is likely to be mediated partially through increased endogenous losses. However, the addition of phytase can partially ameliorate the detrimental effects of IP6 on protein utilization.