A group of ninety-five infants was followed from birth to one year of age, and the total serum IgE concentration was studied. The infants were fed on breast milk, home-prepared cow's milk formula, or proprietary infant milk formula. Solid foods were introduced at 3.5 months of age. The breast milk group had lower IgE than the cow's milk group up to 4 months of age, i.e. until the end of exclusive milk feeding. The formula group had intermediate IgE values. The difference was not due to the presence of IgE antibodies to cow's milk in the bottle-fed infants, since no such antibodies could be detected. After the introduction of solid foods the differences between the groups disappeared. Our data suggest that exclusive breast milk feeding can reduce total serum IgE concentration in early infancy. Reference values for unselected infants and for infants with no atopic manifestations are reported.