The pathogenesis of the granulopoietic failure in three children with severe congenital neutropenia was studied. Mature neutrophils were absent from both peripheral blood and bone marrow. Assay of bone marrow granulocyte colony-forming cells (CFU-C) in a methylcellulose tissue culture system using colony-stimulating activity (CSA) from peripheral blood leukocytes demonstrated normal or increased concentrations of CFU- C compared to those from marrows of 60 age-matched controls. Colonies were of normal size and by light microscopy appeared to contain granulocytes in all stages of maturation including the mature polymorphonuclear neutrophil. CFU-C from peripheral blood of two patients were normal. Production and activity of CSA from the patients' peripheral blood leukocytes and urinary CSA excretion were normal. No serum inhibitors against CFU-C or CSA could be demonstrated using both control and autologous marrow. The defect did not appear to be due to a lack of granulocytic stem cells, a reduction of humoral stimulators of granulopoiesis, nor the presence of an inhibitor as measured by these techniques.