Congenital neutropenia is a lethal disease characterized by recurrent infections beginning in the neonatal period, absence of neutrophils in the peripheral blood, eosinophilia, and monocytosis. The bone marrow shows an apparent "maturation arrest" of the neutrophil series at the promyelocyte stage. Granulocytic colonies grown in vitro in soft agar medium show normal development of eosinophilic colonies and monocyte-macrophage colonies, but defective neutrophil maturation. The abnormal colonies observed contained only myeloblasts and promyelocytes. Thus, it seems to have been possible to mimic in vitro the abnormal differentiation that is observed in vivo.