In addition to meeting nutritional requirements, breast milk plays important roles in biodefense for nursing infants. Dioxins have been detected at high concentrations in breast milk, raising concerns about disorders in nursing infants caused by breast milk containing dioxins in Japan. We analyzed dioxin levels in breast milk and maternal blood samples from 35 pregnant women in Japan. We also measured immunoglobulin (Ig) A concentrations in breast milk and investigated correlations with dioxin concentrations. In addition, 18 of the 35 women took Chlorella pyrenoidosa (Chlorella) supplements during pregnancy, and the effects on dioxin and IgA concentrations in breast milk were investigated. Toxic equivalents were significantly lower in the breast milk of women taking Chlorella tablets than in the Control group (P = .003). These results suggest that Chlorella supplementation by the mother may reduce transfer of dioxins to the child through breast milk. No significant correlation was identified between dioxin and IgA concentrations in breast milk in the Control group. It is unlikely that normal levels of dioxin exposure via food have a remarkable influence on IgA in breast milk. IgA concentrations in breast milk in the Chlorella group were significantly higher than in the Control group (P = .03). Increasing IgA levels in breast milk is considered to be effective for reducing the risk of infection in nursing infants. The present results suggest that Chlorella supplementation not only reduces dioxin levels in breast milk, but may also have beneficial effects on nursing infants by increasing IgA levels in breast milk.