A group of 129 infants were randomly assigned at birth to one of three feeding regimens: human milk (HM), cow's milk formula (CMF) or a casein hydrolysate formula (CHF) during the first 3 days of life. Blood samples were taken on at 4 days and at 2, 4, 8, 12 and 24 months of age. Macromolecular absorption was analysed by measuring the serum concentration of human alpha-lactalbumin (S-alpha-LA) with a competitive radioimmunoassay (RIA). Total serum IgE was measured by RIA. A family history of allergy correlated to the proportion of infants with allergic symptoms both at 24 and 36 months of age (p = 0.03 and p = 0.04 respectively). In none of the three groups did correlation exist between the duration of breastfeeding and serum alpha-LA, serum IgE, family history of allergy, frequency of allergic symptoms and proportion of infants with infections.