Serum IgE in healthy infants was followed longitudinally during the first year of life and was related to heredity for allergy and type of feeding. During the first 6 months of life there were no significant differences of serum IgE levels in relation to presence or absence of heredity for allergy and type of feeding. IgE levels determined at the age of 12 months in infants who were weaned after the age of 6 months were significantly higher than in infants weaned before that age (P<0.05). When the time periods had passed, following onset of cow's milk-feeding and complete cessation of breast-feeding, and were correlated to IgE at the age of 12 months, there were inverse significant correlations; i.e. the earlier the onset of cow's milk-feeding the lower the IgE and the later the cessation of breast-feeding the higher the IgE. This was particularly obvious as regards infants with heredity for allergy and a long duration of breast-feeding (r=−0.43; P<0.01). Infants with heredity for allergy subjected to a hypoallergenic diet adhered to for 6 months had significantly higher IgE at the age of 12 months than infants with similar heredity but a normal diet (P<0.05). The results indicate that serum IgE during late infancy is influenced by both genetic factors and the time of introduction of cow's milk formula in the diet and the time of cessation of breast-feeding.