Aflatoxin M1 is the hydroxylated metabolite of aflatoxin B1, the major analogue of a family of bisfuranocoumarin mycotoxins produced by Aspergillus flavus and related species. Aflatoxin M1 is excreted in the milk of ruminants and is actively monitored in commercial milk production. Aflatoxins are are in fact "pre-toxins", requiring metabolic activation to the toxic principle. The toxicity of these metabolites was first recognised in the 1950s and their structures elucidated in 1963. Aflatoxins have been extensively reviewed.