Aflatoxins are toxic and carcinogenic secondary metabolites produced by some common aspergilli during growth on feeds, foods or laboratory media. Aflatoxin B1 (AFB1) is a decaketide (C20-polyketide) which is synthesized by the mold from acetate units via the polyketide pathway. Methionine contributes the methoxy-methyl group. Six known intermediate compounds in the biosynthesis of AFB1 include norsolorinic acid, averantin, averufin, versiconal hemiacetal acetate, versicolorin A and sterigmatocystin. Other aflatoxins (B2, B2a, G1, G2 and G2a) appear to be conversion products of AFB1. When aflatoxins, and in particular AFB1, occur in feed and are consumed by dairy cattle, a variety of symptoms can occur, which includes unthriftiness, anorexia and decreased milk production. Changes in amounts of enzymes and other blood constituents also result from ingestion of AFB1. The hepatic microsomal mixed-function oxidase system of the cow converts some of the ingested AFB1 into aflatoxin M1 (AFM1), which is excreted in milk. AFM1 retains the toxicity of, but is less carcinogenic than AFB1. Certain heat treatments associated with milk processing appear to inactivate a portion of the AFM1 in milk. If raw milk contains AFM1, products (fluid products, nonfat dried milk, cultured milks, natural cheese, process cheese, butter) made from such milk also will contain AFM1. AFM1 appears to be associated with the casein fraction of milk, hence concentrating the casein in the manufacture of products (e.g. cheese, nonfat dry milk) is accompanied by concentrating of the AFM1. Methods involving thin-layer or high-performance liquid chromatography are commonly used to detect and quantify AFM1 in milk and milk products.