The flow injection spectrophotometric determination of biuret, based on the formation of the copper(II)-biuret complex with an absorption maximum at 550 nm, is described. The relationship between the peak height (absorbance) and the concentration of biuret was linear up to a concentration of 5.0 mg ml^–1. The method is highly selective. Ammonia, which is the only interferent, was removed by a Dowex ion-exchange column built into the flow injection system. The method was applied to the determination of biuret in a commercial urea fertiliser. Fifty samples can be analysed per hour with a relative standard deviation of 1.2–2.5%.