BACKGROUND The precise role of leucocytes in human milk is still unresolved. OBJECTIVE To assist in clarifying the immune mechanisms involved in the development of CMA in suckling infants, we studied the role of immunoregulatory leucocytes and their mediators in human breast milk. METHODS The study population consisted of 43 lactating mothers and their infants, aged 0.25-8.0 months, followed-up prospectively from birth. Of these mothers, 27 had an infant with challenge-proven cow's milk allergy manifested with either skin (n = 23), gastrointestinal (n = 2) or skin and gastrointestinal symptoms (n = 3). Sixteen mothers with a healthy infant served as controls. We evaluated the spontaneous and mitogen-induced tumour necrosis factor-alpha (TNFalpha) and interferon-gamma (IFNgamma) production of human milk leucocytes and isolated peripheral blood lymphocytes in vitro with a commercial ELISA kit. RESULTS TNFalpha production of breast milk leucocytes was significantly lower in the mothers with a cow's milk-allergic infant, whereas IFNgamma production of these cells was comparable in the two groups. CONCLUSION Our results suggest that in the breast milk of mothers having an infant with cow's milk allergy, the number and function of TNFalpha-producing cells is defective. This might lead to a disturbance in the development of oral tolerance and thereby to the development of CMA in suckling infants. These novel results may help in clarifying the etiopathogenesis of CMA.