In 47 infants and children aged 4–66 months with clinically proven cow’s milk allergy and in a group of age-matched controls, serum IgG, IgA and IgM cow’s milk-specific antibodies were determined with ELISA assays while IgE cow’s milk-specific antibodies were measured with Pharmacia RAST. The patients were divided into three separate groups according to the time of clinical response to a standardized cow’s milk challenge protocol. Immediate reactions ( < 45 min after challenge), which were mainly accompanied by urticarial skin eruptions, were associated with elevated IgE milk-specific antibody levels, indicating the involvement of an immediate hypersensitivity mechanism. Alternatively, intermediate reactions (1–20 h after challenge), which were mainly accompanied by vomiting and diarrhea, were not IgE-mediated. In the late reactions ( > 20 h after challenge) both eczematous and gastrointestinal reactions were seen. Patients with eczema-tous eruptions also showed elevated IgE milk-specific antibody levels. IgG milk-specific antibody levels were similar in each of the patient groups but all groups were significantly lower than in the controls. Levels of IgA and IgM milk-specific antibodies were similar in patients and controls. The results indicate that different im-munopathogenic mechanisms are operative in these subgroups of patients with cow’s milk allergy.