We have selected 13 babies presenting with atopic dermatitis during exclusive breast feeding. After evaluation of reaginic reaction, we detected beta-lactoglobulin (BLG) in breast milk and babies sera before and after maternal elimination diet and after maternal challenge. beta-Lactoglobulin was present only in the sera of infants ingesting breast milk containing BLG. We have seen reaginic reactions to cow's milk in seven babies. The maternal elimination diet improved eczema in 11/13 infants, maternal challenge worsened atopic dermatitis in 9/9 children. We demonstrated the transport of food antigen via breast milk into infants' circulation.