After ingestion of food, immune complexes containing food proteins as antigens were demonstrated in the serum of normal and atopic subjects. In the atopic patients challenged with the food to which they were sensitive, abnormal levels were detected. The same atopic patients pretreated with oral sodium cromoglycate had less antigen entry, diminished immune-complex formation, and no atopic symptoms. Tests for antigen entry and immune-complex formation after oral challenge may permit objective assessment of food allergy and may show whether drugs such as oral sodium cromoglycate are likely to benefit individual patients.