Of twenty-five healthy, full term infants without a family history of atopic diseases, 13 were exclusively breast-fed from birth for a minimum of 31/2 months (median 41/2 months), whereas the remaining 12 infants were fed with cow's milk-based formulas from birth for a minimum of 4 months. In the latter group of children a significant increase in serum IgE as well as in salivary IgA was found. In infants exclusively breast-fed, no increase in serum IgE was seen until 6 months of age; at nine months of age, salivary IgA was still significantly lower than in the infants fed cow's milk-based formulas. No children developed obvious allergic diseases during the first three years of life. Thus, cow's milk proteins given to newborn children of non-atopic mothers did not seem to increase the risk of IgE-mediated diseases, maybe due to the development of "blocking" IgA-antibodies in the alimentary tract.