A child is described who had thymic alymphoplasia with increased serum concentrations of γ1M-globulins and a deficiency of γ1A- and 7S γ2-globulins; the child was subject to severe infections from 2 weeks of age and succumbed to pneumonia at 11 months of age. Even though the tissues of this patient at necropsy revealed a remarkable paucity of lymphocytes, there were relatively normal numbers of lymphocytes in his peripheral circulation and his bone marrow. Plasma cells were readily demonstrated in the spleen and lymph nodes. The studies suggest that the development of plasma cells and γ1M-globulin synthesis may occur independently of the development of the thymus and the small lymphocytes in man.