Aflatoxin M 1 (AFM 1 ), the main monohydroxylated of aflatoxin B 1 (AFB 1 ) formed in liver and excreted in the milk, has toxicological proprieties comparable to those of AFB 1 , albeit a lower carcinogenic potency. The presence of AFM 1 was investigated in 30 samples of powdered milk purchased in Argentina and Brazil. The samples were analyzed using an immunoaffinity column for cleanup and HPLC-FLD for determining AFM 1 . The quantification limit was 0.1 μg/kg. AFM 1 was found in all the samples at levels ranging from 0.1 to 0.92 μg kg −1 with average concentration of 0.39 μg kg −1 .