The use of furazolidone in food-producing animals has been banned in the EU. The ban can most effectively be enforced by monitoring for bound residues containing the 3-amino-2-oxazolidinone (AOZ) moiety. Unlike the parent drug, AOZ residues are stable and can be detected for prolonged periods after cessation of treatment. However, AOZ can be passed from pig-to-pig following brief exposure of unmedicated animals to housing that previously contained medicated pigs. We describe criteria by which a distinction may be drawn between pigs treated illegally with the drug and pigs that contain detectable AOZ residues as a result of exposure to contaminated housing. These criteria are that illegally treated pigs will have a concentration ratio of AOZ in bile: kidney of less than 0.3; while unmedicated pigs will have a concentration ratio of AOZ in bile: kidney of greater than 3.0. Using this criteria, 12 pigs, either treated with the drug or exposed to contaminated housing were analysed in a blind study. The pigs were classified as ‘Treated’ or ‘Contaminated’ on the basis of the criteria described above. All 12 pigs were assigned to the correct group. This shows that it is possible to differentiate between furazolidone abuse and contamination.