Enhanced gastrointestinal absorption of dietary substances is an important feature of normal neonatal life that also exists in particular disease states such as selective IgA deficiency and atopic allergy. In these studies, it is shown that patients with hypogammaglobulinemia have increased absorption of dietary bovine antigens and that most patients have large amounts of these proteins present in the serum even after an overnight fast. The amounts of such proteins were found to be correlated with spleen size and/or peripheral lymphoid hypertrophy. Interestingly, three patients with X-linked agammaglobulinemia did not have detectable amounts of these proteins in the serum nor did they have splenomegaly or lymphadenopathy. It is speculated that hypogammaglobulinemic patients have a specific gastrointestinal mucosal lesion that permits the chronic excessive absorption of dietary antigens and may result in lymphoid hypertrophy.