The enzyme phytase catalyses the dephosphorylation of phytic acid and it's salts, phytates. Supplementation of monogastric animal feed with microbial-derived phytase increases the bioavailability of phytic acid bound phosphate. This facilitates a reduction in the addition of inorganic phosphate to the feed and reduces phosphorus excretion. To achieve maximum efficacy in terms of phosphate pollution abatement, supplemental phytases added to animal feed must survive thermal processing of the feed, resist inactivation by the proteolytic enzymes encountered in the animal's digestive tract and display high activity at physiological temperature and pH. A series of in vitro experiments were carried out to determine the relative suitability of four major commercial phytase products for use in animal feed. The enzymes assessed lost between 14% and 72% of their original activities after heating to 80 degrees C for 5 minutes. After exposure to simulated upper digestive tract conditions, the phytases assessed retained between 0 and 28% of their original activities. The commercial phytases displayed between 98% and 67% of their maximum activities at 39 degrees C and all phytases assessed had an optimum pH between pH 4 and pH 5. None of the phytases assessed satisfied all of the criteria of an ideal phytase for use in animal feed.