Abstract Stimulation ([ 3H]thymidine incorporation) of blood lymphocytes cultured with food proteins was evaluated in infants with food protein-induced enterocolitis and correlated with the results of oral diagnostic challenges with the same foods (soy, cow's milk, and egg white). The geometric mean stimulation index for lymphocytes from patients with positive oral soy protein challenge that were cultured with soy protein was 8.5, and for patients with positive cow's milk challenge the stimulation index was 6.0 when casein was used in the cultures. Both values are significantly different from the values obtained from patients with negative oral challenges (p < 0.01). The enhanced lymphocyte responses were specific for the food proteins responsible for clinical symptoms. It is not clear whether these lymphocyte responses are due to systemic immunization secondary to macromolecular absorption, or to an abnormality in immune regulation such as a delay in the development of oral tolerance mechanisms. They suggest, however, that circulating lymphocytes sensitive to the food antigens that produce the clinical symptoms are frequent in infants with this discrete form of food protein hypersensitivity.