Three groups of infants were formed according to the type of feeding: I-breast milk from mothers without any kind of elimination diet, II-cow's milk-based formula, and III-breast milk till 2 months of age and after that, cow's milk formula. CD4 and CD8 were determined by monoclonal antibodies technique, IgE by ELISA and prick test with either intact or enzymatically digested cow's milk antigens. The average age of onset of the atopic disease in the first group was 2m 10d +/- 15d, in the second: 3m 4d +/- 14d, and in the third: 2m 11d +/- 1m 18d. The number of CD4 in group II was the lowest while in group III was the highest. CD8 value in the second group was higher than in the others. Ratio between CD4 and CD8 in cow's milk-fed infants was the lowest. Total serum IgE level in group II was the lowest although the mean values of the three groups were significantly increased. Prick tests, with at least one antigen, were positive in 75%, 50% and 48% in group I, II and III respectively. It is suggested that the behaviour of the parameters studied would be related to the dose of cow's milk proteins received through breast milk in breast-fed infants; to the relatively great amount of cow's milk antigens and to the lack of breast-feeding in infants fed on cow's milk formula; and to the interaction of such factors in infants fed with both cow's milk and breast milk.