The biuretolytic activity of the ruminal flora which developed in the ingesta of Merino sheep in response to feeding biuret with practical rations was demonstrated. The activity was determined by measuring the rate of disappearance of biuret in an in vitro fermentation system. Biuret was determined by a specific, semi-micro, colorimetric method based on the formation of a yellow nickel-biuret complex and adapted for use with ruminal fluid. The activity was evoked only when biuret was included in diets of poor quality teff hay with or without maize meal. With ruminal ingesta from sheep fed teff hay and biuret the activity was low, but when maize meal was given in addition the activity was about three times as high, and sufficient to account for the disappearance of up to 16-4 g daily in Merino sheep with a volume of ruminal ingesta of 6 1. The course of development of biuretolytic activity is discussed in relation to retention of biuret-nitrogen.