Summary and Conclusions1. The activity or concentration of the clotting factors recognized at the present time have been determined in 7 patients with agammaglobulinemia. Four of the patients had the congenital-hereditary form of agammaglobulinemia and three had the acquired form of the disease. 2. An elevated platelet count was observed in 5 of the 7 patients. No abnormalities in plasma clotting factors were found. 3. Data presented are consistent with the following conclusions: (a) none of the plasma coagulation factors is a gamma globulin, (b) agammaglobulinemia is probably the result of an isolated deficiency in protein synthesis, c) the deficiency of gamma globulin synthesis in these patients does not involve the liver but rather depends on an anomaly of protein metabolism existing elsewhere in the reticuloendothelial system.