This study examined the exposure of infants to aflatoxin M1 (AFM1) and of lactating mothers to aflatoxin B1 (AFB1), using AFM1 in breast milk as a biomarker for exposure to AFB1. An enzyme-linked immunosorbent assay was modified for the analysis of AFM1 in breast milk samples from 73 women from Victoria for comparison with breast milk samples from Thailand (n = 11). The results were compared with those obtained by HPLC. AFM1 was detected in 11 samples from Victoria and five samples from Thailand at median concentrations of 0.071 ng/ml (range 0.028 to 1.031 ng/ml) and 0.664 ng/ml (range 0.039 to 1.736 ng/ml), respectively. Levels of AFM1 in Thai milk samples were significantly higher than those in milk samples from Victoria.