This International Standard specifies a method for the determination of the lactulose content of heated milk, skimmed, partially skimmed or whole milk, by high-performance liquid chromatography, in order to distinguish milk sterilized by ultra-heat treatment (UHT) from in-bottle sterilized milk. The method has been tested over a lactulose content range of 200 mg/l to 1 500 mg/l and is applicable to all heat-treated milks. The method described in this International Standard is to be used in cases of dispute.