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Background and purpose: Aflatoxin B1 is the most important poison of aflatoxins that contaminated animal feeds by mold and is transformed to type M1 in kidney and liver after some chemical reactions and they are transmitted to human through animal milk Materials and methods: In current study, 75 samples of pasteurized milk were collected from Mazandaran Province during spring and summer of 2011 and amount of aflatoxin M1was measured by ELISA method. T-test was used for mean comparison Results: Results showed that aflatoxin M1in 4% samples (3.75) was non dedtected and in 37.3% samples (27.75) concentration of aflatoxin M1 was less than the limit permitted by Codex. However, 62.67% samples (47.75) had a contamination of aflatoxin M1 higher than Codex threshold level, among which 12% samples (9.75) had even higher concentration than Iran's national standard permit. The highest amount was observed in factory NO.21 (97.55 ng/l). There was no significant relation between seasons and aflatoxin Conclusion: Due to high level of contamination observed in samples, regular monitoring of contamination in milk samples and controlling of most contaminating causes seem to be necessary