55 infants, aged between 1 and 13 months, who presented with persistent diarrhoea were placed on a diet free of cows' milk protein. The clinical condition of the infants improved and 6-8 weeks later each was challenged with low-lactose cows' milk. The following investigations were performed before and 24 hours after provocation: jejunal biopsy, serum and duodenal juice levels of complement C3 and C4, C3-activator, and immunoglobulins A, G, M, and D. Three groups were recognised on postchallenge clinical symptoms together with histological changes in the jejunal mucosa: group 1 (n=10 infants) with no clinical or mucosal abnormality, group 2 (n=18 infants) with mucosal abnormalities but lacking clinical symptoms, and group 3 (n=27 infants) with mucosal abnormalities and clinical symptoms; but in group 3 symptoms were delayed for over 24 hours in 7 infants, and in the remaining 20 infants symptoms appeared within 24 hours. After provocation the serum IgG levels were decreased in 3 infants in group 1, 12 in group 2, and 17 in group 3. The mean serum IgA and IgM levels were higher in group 1 compared with group 2 or 3; pre- and postchallenge mean serum immunoglobulin levels, complement, and complement activator levels were of equal magnitude in all three groups. A depletion of serum complement C3 level was observed in 4 (group 1), 4 (group 2), and 13 (group 3) infants 24 hours after provocation with cows' milk; the fall in serum IgG was often associated with depletion of complement in serum. This suggests the involvement in some infants of IgG-mediated complement-consuming immune reactions in the development of the damage in the intestinal mucosa. After challenge, exudation of IgG and IgA occurred irrespective of the presence or absence of mucosal damage and was not always associated with the decrease in serum IgG concentration. Because of the variable conditions present in the intestinal lumen it is suggested that the level of immunoglobulins in duodenal juice is not a reliable measure of the local immune response. Levels of complement and immunoglobulin in serum or duodenal juice fail to provide help in the diagnosis of cows' milk protein-sensitive enteropathy.