During surveillance of hog carcasses from Manitoba for antibiotic residues by the Health of Animals Laboratory, Agriculture Canada, Saskatoon, an unknown substance was found which produced tetracycline‐like results with the methods used. This same substance was found in an implicated swine feed premix. Using various HPLC systems and columns, UV spectroscopy, reverse‐phase TLC, and mass spectrometry, the substance was isolated from the feed premix, and identified as lumichrome, a photodegradation product of riboflavin. Traces of the same substance were found in riboflavin standard. Analysis of swine kidney, previously found to contain the unknown, showed the same substance was present at a level of about 1 ppm.