The steps are presented from the discovery of aflatoxin M1 in the milk of a cow fed “toxic” peanut meal to the determination of its relation to aflatoxin B1 and the relation of aflatoxin M1 to other animal metabolites of aflatoxin B1. Estimates from controlled studies are given of the level and amount of aflatoxin M1 to be expected in the milk of cows exposed to aflatoxin B1 in their feed, followed by numerous survey results that corroborate the conclusion from the experimental estimates: levels of aflatoxin B1 in uncontrolled dairy ration ingredients are sufficiently high to result in measurable aflatoxin M1 ( > 0.1 ng/ml) in commercial milk supplies. Because the Food and Drug Administration has been unable to prevent occurrence of aflatoxin M1 in milk in the United States by attempting to control the feed, an administrative guideline has been established at 0.5 ppb for aflatoxin M1 in fluid milk products; consideration is being given to related guidelines for products manufactured from milk.