Yucca schidigera extracts have been reported to reduce noxious gases such as ammonia in animal rearing facilities when incorporated into the diet. Ammonia reduction effects have been hypothesised to be due to inhibition of microbial urease by the extract The effect of a commercial extract, (De-Odorase, Alltech Inc., Nicholasville, Kentucky, U.S.A.), on the activity of urease enzyme from Bacillus pasteurii was determined by measuring the ammonia generated from the breakdown of urea using a modified Berthelot assay. The basis of this assay is the reaction of ammonia, hypochlorite and phenol to give phenolindophenol dye which can be measured colorimetrically. The use of trichloroacetic acid as a stopping reagent allowed addition of extract to controls and standards after enzyme reaction but before colour development. As ionic strength has been reported to affect the activity of certain ureases, it was fixed at 0.90 for all enzyme reactions. Subsaturating substrate concentrations (5 mM) were employed to mimic in vivo conditions. Reaction velocity was measured over the pH range 3.0 to 8.5 in 170 mM potassium citrate, potassium phosphate and potassium HEPES buffers in 0.4% extract (40 times typical dosage). Over the active pH range of the enzyme no significant differences between control and analytical urease activities were observed (P > 0.10). Phosphate strongly inhibited this particular urease, as has been reported for C.ensformis urease and was deemed unsuitable for such assays. We conclude that it is very unlikely that Y.schidigera extracts function as urease inhibitors either in vitro or in vivo.