The metabolic product of aflatoxin B1 that appears in the milk of cattle and other mammals fed B1-contaminated rations — the liver hydroxylates B1 and the resulting M1 is excreted in milk. Though less toxic than B1, it retains significant carcinogenic potential, and because milk is a staple for children, regulators set especially tight limits for M1, far below grain tolerances. It survives pasteurisation and processing into dairy products, so control depends entirely on keeping dairy feed free of aflatoxin: feed testing is the critical defence, since no downstream step removes it.