Eight out of 22 patients with aplastic anemia died from infections. All five patients with fewer than 100 granulocytes and 20 monocytes per cu mm of blood succumbed to overwhelming sepsis. The relationship that existed between absolute granulocyte and monocyte counts made it impossible to determine whether neutropenia or monocytopenia carried the greater risk. Monocytopenia correlated with reduced numbers of phagocytic cells in WBC cultures. This lack of phagocytes was associated with impairment of macrophage-dependent reactions, e.g., the mixed WBC reaction. The data suggest that aplastic anemia reduces the delivery of macrophage precursors into the circulation; this could ultimately deplete tissues of macrophages. Thus, monocytopenia, as well as neutropenia, may contribute to the risk from infection in patients with bone marrow failure.