One hundred samples of commercially available cows milk, collected in the state of São Paulo, from July 1979 to September 1981, were analysed to determine the levels of aflatoxins M1and M2by the method of the AOAC. This investigation was also undertaken in 50 samples of cows milk from two farms located in the Médio Vale do Paraiba, from animals which had ingested stored feed. Aflatoxin M1was detected in only one sample of commercially available cows milk, while those from the farms were found to contain a minimum of 0.1 μg/1 and a maximum of 1.68 μg/l.